diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index d64b9085e3..2af6e56a3d 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -290,6 +290,13 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
       }
     }
   } else {
+    // Unlike check_split_disallowed_err, ignore `split_disallowed`, as opening a float shouldn't
+    // mess with the frame structure. Still check `b_locked_split` to avoid opening more windows
+    // into a closing buffer, though.
+    if (curwin->w_buffer->b_locked_split) {  // Can't instead check `buf` in case win_set_buf fails!
+      api_set_error(err, kErrorTypeException, "E1159: Cannot open a float when closing the buffer");
+      goto cleanup;
+    }
     wp = win_new_float(NULL, false, fconfig, err);
   }
   if (!wp) {
@@ -336,6 +343,7 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     }
   }
   if (!tp) {
+    api_clear_error(err);  // may have been set by win_set_buf
     api_set_error(err, kErrorTypeException, "Window was closed immediately");
     goto cleanup;
   }
@@ -489,7 +497,14 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     if (curwin_moving_tp) {
       if (was_split) {
         int dir;
-        win_goto(winframe_find_altwin(win, &dir, NULL, NULL));
+        win_T *altwin = winframe_find_altwin(win, &dir, NULL, NULL);
+        // Autocommands may still make this the last non-float after this check.
+        // That case will be caught later when trying to move the window.
+        if (!altwin) {
+          api_set_error(err, kErrorTypeException, "Cannot move last non-floating window");
+          return;
+        }
+        win_goto(altwin);
       } else {
         win_goto(win_float_find_altwin(win, NULL));
       }
@@ -522,7 +537,7 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
         // FIXME(willothy): if the window is the last in the tabpage but there is another tabpage
         // and the target window is in that other tabpage, should we move the window to that
         // tabpage and close the previous one, or just error?
-        api_set_error(err, kErrorTypeException, "Cannot move last window");
+        api_set_error(err, kErrorTypeException, "Cannot move last non-floating window");
         goto restore_curwin;
       } else if (parent != NULL && parent->handle == win->handle) {
         int n_frames = 0;
diff --git a/src/nvim/api/window.c b/src/nvim/api/window.c
index cde629dab3..2bdb11d285 100644
--- a/src/nvim/api/window.c
+++ b/src/nvim/api/window.c
@@ -374,7 +374,7 @@ void nvim_win_hide(Window window, Error *err)
     } else if (tabpage == curtab) {
       win_close(win, false, false);
     } else {
-      win_close_othertab(win, false, tabpage);
+      win_close_othertab(win, false, tabpage, false);
     }
   });
 }
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 8c0fb9a230..4d46a3ec7c 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -500,7 +500,7 @@ static bool can_unload_buffer(buf_T *buf)
 ///               close all other windows.
 /// @param ignore_abort
 ///               If true, don't abort even when aborting() returns true.
-/// @return  true when we got to the end and b_nwindows was decremented.
+/// @return  true if b_nwindows was decremented directly by this call (e.g: not via autocmds).
 bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool ignore_abort)
 {
   bool unload_buf = (action != 0);
@@ -571,7 +571,7 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
     }
     buf->b_locked--;
     buf->b_locked_split--;
-    if (abort_if_last && one_window(win)) {
+    if (abort_if_last && win != NULL && one_window(win, NULL)) {
       // Autocommands made this the only window.
       emsg(_(e_auabort));
       return false;
@@ -590,7 +590,7 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
       }
       buf->b_locked--;
       buf->b_locked_split--;
-      if (abort_if_last && one_window(win)) {
+      if (abort_if_last && win != NULL && one_window(win, NULL)) {
         // Autocommands made this the only window.
         emsg(_(e_auabort));
         return false;
@@ -625,7 +625,7 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
   // Return when a window is displaying the buffer or when it's not
   // unloaded.
   if (buf->b_nwindows > 0 || !unload_buf) {
-    return false;
+    return true;
   }
 
   if (buf->terminal) {
@@ -702,7 +702,7 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
     }
     // Do not wipe out the buffer if it is used in a window.
     if (buf->b_nwindows > 0) {
-      return false;
+      return true;
     }
     FOR_ALL_TAB_WINDOWS(tp, wp) {
       mark_forget_file(wp, buf->b_fnum);
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 0a52b7e909..a3abe75150 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -4915,7 +4915,7 @@ void ex_win_close(int forceit, win_T *win, tabpage_T *tp)
   if (tp == NULL) {
     win_close(win, !need_hide && !buf_hide(buf), forceit);
   } else {
-    win_close_othertab(win, !need_hide && !buf_hide(buf), tp);
+    win_close_othertab(win, !need_hide && !buf_hide(buf), tp, forceit);
   }
 }
 
diff --git a/src/nvim/version.c b/src/nvim/version.c
index 755288f137..400a9719bf 100644
--- a/src/nvim/version.c
+++ b/src/nvim/version.c
@@ -2720,7 +2720,7 @@ bool may_show_intro(void)
           && (curbuf->b_fname == NULL)
           && (curbuf->handle == 1)
           && (curwin->handle == LOWEST_WIN_ID)
-          && one_window(curwin)
+          && one_window(curwin, NULL)
           && (vim_strchr(p_shm, SHM_INTRO) == NULL));
 }
 
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 7cfe436337..9e36d93953 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -427,7 +427,7 @@ newwindow:
   // move window to new tab page
   case 'T':
     CHECK_CMDWIN;
-    if (one_window(curwin)) {
+    if (one_window(curwin, NULL)) {
       msg(_(m_onlyone), 0);
     } else {
       tabpage_T *oldtab = curtab;
@@ -500,7 +500,7 @@ newwindow:
   case 'H':
   case 'L':
     CHECK_CMDWIN;
-    if (one_window(curwin)) {
+    if (one_window(curwin, NULL)) {
       beep_flush();
     } else {
       const int dir = ((nchar == 'H' || nchar == 'L') ? WSP_VERT : 0)
@@ -754,15 +754,17 @@ static void cmd_with_count(char *cmd, char *bufp, size_t bufsize, int64_t Prenum
 void win_set_buf(win_T *win, buf_T *buf, Error *err)
   FUNC_ATTR_NONNULL_ALL
 {
+  const handle_T win_handle = win->handle;
   tabpage_T *tab = win_find_tabpage(win);
 
   // no redrawing and don't set the window title
   RedrawingDisabled++;
 
   switchwin_T switchwin;
+  int win_result;
 
   TRY_WRAP(err, {
-    int win_result = switch_win_noblock(&switchwin, win, tab, true);
+    win_result = switch_win_noblock(&switchwin, win, tab, true);
     if (win_result != FAIL) {
       const int save_acd = p_acd;
       if (!switchwin.sw_same_win) {
@@ -777,6 +779,9 @@ void win_set_buf(win_T *win, buf_T *buf, Error *err)
       }
     }
   });
+  if (win_result == FAIL && !ERROR_SET(err)) {
+    api_set_error(err, kErrorTypeException, "Failed to switch to window %d", win_handle);
+  }
 
   // If window is not current, state logic will not validate its cursor. So do it now.
   // Still needed if do_buffer returns FAIL (e.g: autocmds abort script after buffer was set).
@@ -1081,7 +1086,7 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
   bool toplevel = flags & (WSP_TOP | WSP_BOT);
 
   // add a status line when p_ls == 1 and splitting the first window
-  if (one_window(firstwin) && p_ls == 1 && oldwin->w_status_height == 0) {
+  if (one_window(firstwin, NULL) && p_ls == 1 && oldwin->w_status_height == 0) {
     if (oldwin->w_height <= p_wmh) {
       emsg(_(e_noroom));
       return NULL;
@@ -1791,7 +1796,7 @@ static void win_exchange(int Prenum)
     return;
   }
 
-  if (one_window(curwin)) {
+  if (one_window(curwin, NULL)) {
     // just one window
     beep_flush();
     return;
@@ -1883,7 +1888,7 @@ static void win_rotate(bool upwards, int count)
     return;
   }
 
-  if (count <= 0 || one_window(curwin)) {
+  if (count <= 0 || one_window(curwin, NULL)) {
     // nothing to do
     beep_flush();
     return;
@@ -1966,7 +1971,7 @@ int win_splitmove(win_T *wp, int size, int flags)
   int dir = 0;
   int height = wp->w_height;
 
-  if (one_window(wp)) {
+  if (one_window(wp, NULL)) {
     return OK;  // nothing to do
   }
   if (is_aucmd_win(wp) || check_split_disallowed(wp) == FAIL) {
@@ -2495,7 +2500,7 @@ void close_windows(buf_T *buf, bool keep_curwin)
 
   // Start from lastwin to close floating windows with the same buffer first.
   // When the autocommand window is involved win_close() may need to print an error message.
-  for (win_T *wp = lastwin; wp != NULL && (is_aucmd_win(lastwin) || !one_window(wp));) {
+  for (win_T *wp = lastwin; wp != NULL && (is_aucmd_win(lastwin) || !one_window(wp, NULL));) {
     if (wp->w_buffer == buf && (!keep_curwin || wp != curwin)
         && !(win_locked(wp) || wp->w_buffer->b_locked > 0)) {
       if (win_close(wp, false, false) == FAIL) {
@@ -2520,7 +2525,10 @@ void close_windows(buf_T *buf, bool keep_curwin)
       for (win_T *wp = tp->tp_lastwin; wp != NULL; wp = wp->w_prev) {
         if (wp->w_buffer == buf
             && !(win_locked(wp) || wp->w_buffer->b_locked > 0)) {
-          win_close_othertab(wp, false, tp);
+          if (!win_close_othertab(wp, false, tp, false)) {
+            // If closing the window fails give up, to avoid looping forever.
+            break;
+          }
 
           // Start all over, the tab page may be closed and
           // autocommands may change the window layout.
@@ -2537,27 +2545,30 @@ void close_windows(buf_T *buf, bool keep_curwin)
 /// Check if "win" is the last non-floating window that exists.
 bool last_window(win_T *win) FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  return one_window(win) && first_tabpage->tp_next == NULL;
+  return one_window(win, NULL) && first_tabpage->tp_next == NULL;
 }
 
-/// Check if "win" is the only non-floating window in the current tabpage.
+/// Check if "win" is the only non-floating window in tabpage "tp", or NULL for current tabpage.
 ///
 /// This should be used in place of ONE_WINDOW when necessary,
 /// with "firstwin" or the affected window as argument depending on the situation.
-bool one_window(win_T *win) FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
+bool one_window(win_T *win, tabpage_T *tp)
+  FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ARG(1)
 {
-  assert(!firstwin->w_floating);
-  return firstwin == win && (win->w_next == NULL || win->w_next->w_floating);
+  win_T *first = tp ? tp->tp_firstwin : firstwin;
+  assert((!tp || tp != curtab) && !first->w_floating);
+  return first == win && (win->w_next == NULL || win->w_next->w_floating);
 }
 
-/// Check if floating windows in the current tab can be closed.
+/// Check if floating windows in tabpage `tp` can be closed.
 /// Do not call this when the autocommand window is in use!
 ///
+/// @param tp tabpage to check. Must be NULL for the current tabpage.
 /// @return true if all floating windows can be closed
-static bool can_close_floating_windows(void)
+static bool can_close_floating_windows(tabpage_T *tp)
 {
-  assert(!is_aucmd_win(lastwin));
-  for (win_T *wp = lastwin; wp->w_floating; wp = wp->w_prev) {
+  assert(tp != curtab && (tp || !is_aucmd_win(lastwin)));
+  for (win_T *wp = tp ? tp->tp_lastwin : lastwin; wp->w_floating; wp = wp->w_prev) {
     buf_T *buf = wp->w_buffer;
     int need_hide = (bufIsChanged(buf) && buf->b_nwindows <= 1);
 
@@ -2618,10 +2629,10 @@ static bool close_last_window_tabpage(win_T *win, bool free_buf, tabpage_T *prev
   // case they have already been triggered.
   goto_tabpage_tp(alt_tabpage(), false, win->w_buffer != NULL);
 
-  // Safety check: Autocommands may have closed the window when jumping
-  // to the other tab page.
-  if (valid_tabpage(prev_curtab) && prev_curtab->tp_firstwin == win) {
-    win_close_othertab(win, free_buf, prev_curtab);
+  // Safety check: Autocommands may have switched back to the old tab page
+  // or closed the window when jumping to the other tab page.
+  if (curtab != prev_curtab && valid_tabpage(prev_curtab) && prev_curtab->tp_firstwin == win) {
+    win_close_othertab(win, free_buf, prev_curtab, false);
   }
   entering_window(curwin);
 
@@ -2639,7 +2650,9 @@ static bool close_last_window_tabpage(win_T *win, bool free_buf, tabpage_T *prev
 /// "action" can also be zero (do nothing).
 /// "abort_if_last" is passed to close_buffer(): abort closing if all other
 /// windows are closed.
-static void win_close_buffer(win_T *win, int action, bool abort_if_last)
+///
+/// @return  whether close_buffer() decremented b_nwindows
+static bool win_close_buffer(win_T *win, int action, bool abort_if_last)
   FUNC_ATTR_NONNULL_ALL
 {
   // Free independent synblock before the buffer is freed.
@@ -2654,12 +2667,13 @@ static void win_close_buffer(win_T *win, int action, bool abort_if_last)
     win->w_buffer->b_p_bl = false;
   }
 
+  bool retval = false;
   // Close the link to the buffer.
   if (win->w_buffer != NULL) {
     bufref_T bufref;
     set_bufref(&bufref, curbuf);
     win->w_locked = true;
-    close_buffer(win, win->w_buffer, action, abort_if_last, true);
+    retval = close_buffer(win, win->w_buffer, action, abort_if_last, true);
     if (win_valid_any_tab(win)) {
       win->w_locked = false;
     }
@@ -2670,6 +2684,27 @@ static void win_close_buffer(win_T *win, int action, bool abort_if_last)
       curbuf = firstbuf;
     }
   }
+
+  return retval;
+}
+
+/// When failing to close a window after already calling close_buffer() on it,
+/// call this to make the window have a buffer again.
+///
+/// @param bufref         reference to win->w_buffer before calling close_buffer()
+/// @param did_decrement  whether close_buffer() decremented b_nwindows
+static void win_unclose_buffer(win_T *win, bufref_T *bufref, bool did_decrement)
+{
+  if (win->w_buffer == NULL) {
+    // If the buffer was removed from the window we have to give it any buffer.
+    win->w_buffer = firstbuf;
+    firstbuf->b_nwindows++;
+    win_init_empty(win);
+  } else if (did_decrement && win->w_buffer == bufref->br_buf && bufref_valid(bufref)) {
+    // close_buffer() decremented the window count, but we're keeping the window.
+    // As the window is still viewing the buffer, increment the count.
+    win->w_buffer->b_nwindows++;
+  }
 }
 
 // Close window "win".  Only works for the current tab page.
@@ -2697,12 +2732,12 @@ int win_close(win_T *win, bool free_buf, bool force)
     emsg(_(e_autocmd_close));
     return FAIL;
   }
-  if (lastwin->w_floating && one_window(win)) {
+  if (lastwin->w_floating && one_window(win, NULL)) {
     if (is_aucmd_win(lastwin)) {
       emsg(_("E814: Cannot close window, only autocmd window would remain"));
       return FAIL;
     }
-    if (force || can_close_floating_windows()) {
+    if (force || can_close_floating_windows(NULL)) {
       // close the last window until the there are no floating windows
       while (lastwin->w_floating) {
         // `force` flag isn't actually used when closing a floating window.
@@ -2793,7 +2828,10 @@ int win_close(win_T *win, bool free_buf, bool force)
     return OK;
   }
 
-  win_close_buffer(win, free_buf ? DOBUF_UNLOAD : 0, true);
+  bufref_T bufref;
+  set_bufref(&bufref, win->w_buffer);
+
+  bool did_decrement = win_close_buffer(win, free_buf ? DOBUF_UNLOAD : 0, true);
 
   if (win_valid(win) && win->w_buffer == NULL
       && !win->w_floating && last_window(win)) {
@@ -2808,14 +2846,23 @@ int win_close(win_T *win, bool free_buf, bool force)
   if (curtab != prev_curtab && win_valid_any_tab(win)
       && win->w_buffer == NULL) {
     // Need to close the window anyway, since the buffer is NULL.
-    win_close_othertab(win, false, prev_curtab);
+    win_close_othertab(win, false, prev_curtab, force);
     return FAIL;
   }
 
   // Autocommands may have closed the window already, or closed the only
   // other window or moved to another tab page.
-  if (!win_valid(win) || (!win->w_floating && last_window(win))
-      || close_last_window_tabpage(win, free_buf, prev_curtab)) {
+  if (!win_valid(win)) {
+    return FAIL;
+  }
+  if (one_window(win, NULL) && (first_tabpage->tp_next == NULL || lastwin->w_floating)) {
+    if (first_tabpage->tp_next != NULL) {
+      emsg(e_floatonly);
+    }
+    win_unclose_buffer(win, &bufref, did_decrement);
+    return FAIL;
+  }
+  if (close_last_window_tabpage(win, free_buf, prev_curtab)) {
     return FAIL;
   }
 
@@ -2974,14 +3021,43 @@ static void do_autocmd_winclosed(win_T *win)
 // thus "tp" may become invalid!
 // Caller must check if buffer is hidden and whether the tabline needs to be
 // updated.
-void win_close_othertab(win_T *win, int free_buf, tabpage_T *tp)
+// @return false when the window was not closed as a direct result of this call
+//         (e.g: not via autocmds).
+bool win_close_othertab(win_T *win, int free_buf, tabpage_T *tp, bool force)
   FUNC_ATTR_NONNULL_ALL
 {
+  assert(tp != curtab);
+  bool did_decrement = false;
+
   // Get here with win->w_buffer == NULL when win_close() detects the tab page
   // changed.
   if (win_locked(win)
       || (win->w_buffer != NULL && win->w_buffer->b_locked > 0)) {
-    return;  // window is already being closed
+    return false;  // window is already being closed
+  }
+  if (is_aucmd_win(win)) {
+    emsg(_(e_autocmd_close));
+    return false;
+  }
+
+  // Check if closing this window would leave only floating windows.
+  if (tp->tp_lastwin->w_floating && one_window(win, tp)) {
+    if (force || can_close_floating_windows(tp)) {
+      // close the last window until the there are no floating windows
+      while (tp->tp_lastwin->w_floating) {
+        // `force` flag isn't actually used when closing a floating window.
+        if (!win_close_othertab(tp->tp_lastwin, free_buf, tp, true)) {
+          // If closing the window fails give up, to avoid looping forever.
+          goto leave_open;
+        }
+      }
+      if (!win_valid_any_tab(win)) {
+        return false;  // window already closed by autocommands
+      }
+    } else {
+      emsg(e_floatonly);
+      goto leave_open;
+    }
   }
 
   // Fire WinClosed just before starting to free window-related resources.
@@ -2991,87 +3067,83 @@ void win_close_othertab(win_T *win, int free_buf, tabpage_T *tp)
     do_autocmd_winclosed(win);
     // autocmd may have freed the window already.
     if (!win_valid_any_tab(win)) {
-      return;
+      return false;
     }
   }
 
+  bufref_T bufref;
+  set_bufref(&bufref, win->w_buffer);
+
   if (win->w_buffer != NULL) {
     // Close the link to the buffer.
-    close_buffer(win, win->w_buffer, free_buf ? DOBUF_UNLOAD : 0, false, true);
+    did_decrement = close_buffer(win, win->w_buffer, free_buf ? DOBUF_UNLOAD : 0, false, true);
   }
 
-  tabpage_T *ptp = NULL;
-
   // Careful: Autocommands may have closed the tab page or made it the
   // current tab page.
-  for (ptp = first_tabpage; ptp != NULL && ptp != tp; ptp = ptp->tp_next) {}
-  if (ptp == NULL || tp == curtab) {
-    // If the buffer was removed from the window we have to give it any
-    // buffer.
-    if (win_valid_any_tab(win) && win->w_buffer == NULL) {
-      win->w_buffer = firstbuf;
-      firstbuf->b_nwindows++;
-      win_init_empty(win);
-    }
-    return;
+  if (!valid_tabpage(tp) || tp == curtab) {
+    goto leave_open;
+  }
+  // Autocommands may have closed the window already, or nvim_win_set_config
+  // moved it to a different tab page.
+  if (!tabpage_win_valid(tp, win)) {
+    goto leave_open;
+  }
+  // Autocommands may again cause closing this window to leave only floats.
+  // Check again; we'll not bother closing floating windows this time.
+  if (tp->tp_lastwin->w_floating && one_window(win, tp)) {
+    emsg(e_floatonly);
+    goto leave_open;
   }
 
-  // Autocommands may have closed the window already.
-  {
-    bool found_window = false;
-    FOR_ALL_WINDOWS_IN_TAB(wp, tp) {
-      if (wp == win) {
-        found_window = true;
-        break;
-      }
-    }
-    if (!found_window) {
-      return;
-    }
-  }
-
-  bool free_tp = false;
+  int free_tp_idx = 0;
 
   // When closing the last window in a tab page remove the tab page.
   if (tp->tp_firstwin == tp->tp_lastwin) {
-    char prev_idx[NUMBUFLEN];
-    if (has_event(EVENT_TABCLOSED)) {
-      vim_snprintf(prev_idx, NUMBUFLEN, "%i", tabpage_index(tp));
-    }
-
+    free_tp_idx = tabpage_index(tp);
     int h = tabline_height();
 
     if (tp == first_tabpage) {
       first_tabpage = tp->tp_next;
     } else {
+      tabpage_T *ptp;
       for (ptp = first_tabpage; ptp != NULL && ptp->tp_next != tp;
            ptp = ptp->tp_next) {
         // loop
       }
       if (ptp == NULL) {
         internal_error("win_close_othertab()");
-        return;
+        return false;
       }
       ptp->tp_next = tp->tp_next;
     }
-    free_tp = true;
     redraw_tabline = true;
     if (h != tabline_height()) {
       win_new_screen_rows();
     }
-
-    if (has_event(EVENT_TABCLOSED)) {
-      apply_autocmds(EVENT_TABCLOSED, prev_idx, prev_idx, false, win->w_buffer);
-    }
   }
 
   // Free the memory used for the window.
+  buf_T *buf = win->w_buffer;
   int dir;
   win_free_mem(win, &dir, tp);
 
-  if (free_tp) {
+  if (free_tp_idx > 0) {
     free_tabpage(tp);
+
+    if (has_event(EVENT_TABCLOSED)) {
+      char prev_idx[NUMBUFLEN];
+      vim_snprintf(prev_idx, NUMBUFLEN, "%i", free_tp_idx);
+      apply_autocmds(EVENT_TABCLOSED, prev_idx, prev_idx, false, buf);
+    }
   }
+  return true;
+
+leave_open:
+  if (win_valid_any_tab(win)) {
+    win_unclose_buffer(win, &bufref, did_decrement);
+  }
+  return false;
 }
 
 /// Free the memory used for a window.
@@ -3219,14 +3291,15 @@ win_T *winframe_remove(win_T *win, int *dirp, tabpage_T *tp, frame_T **unflat_al
 /// @param tp    tab page "win" is in, NULL for current
 /// @param altfr if not NULL, set to pointer of frame that will get the space
 ///
-/// @return      a pointer to the window that will get the freed up space.
+/// @return      a pointer to the window that will get the freed up space, or NULL
+///              if there is no other non-float to receive the space.
 win_T *winframe_find_altwin(win_T *win, int *dirp, tabpage_T *tp, frame_T **altfr)
   FUNC_ATTR_NONNULL_ARG(1, 2)
 {
   assert(tp == NULL || tp != curtab);
 
-  // If there is only one window there is nothing to remove.
-  if (tp == NULL ? ONE_WINDOW : tp->tp_firstwin == tp->tp_lastwin) {
+  // If there is only one non-floating window there is nothing to remove.
+  if (one_window(win, tp)) {
     return NULL;
   }
 
@@ -3423,7 +3496,7 @@ static frame_T *win_altframe(win_T *win, tabpage_T *tp)
 {
   assert(tp == NULL || tp != curtab);
 
-  if (tp == NULL ? ONE_WINDOW : tp->tp_firstwin == tp->tp_lastwin) {
+  if (one_window(win, tp)) {
     return alt_tabpage()->tp_curwin->w_frame;
   }
 
@@ -3992,7 +4065,7 @@ void close_others(int message, int forceit)
     return;
   }
 
-  if (one_window(firstwin) && !lastwin->w_floating) {
+  if (one_window(firstwin, NULL) && !lastwin->w_floating) {
     if (message
         && !autocmd_busy) {
       msg(_(m_onlyone), 0);
@@ -7049,7 +7122,7 @@ int global_stl_height(void)
 /// @param morewin  pretend there are two or more windows if true.
 int last_stl_height(bool morewin)
 {
-  return (p_ls > 1 || (p_ls == 1 && (morewin || !one_window(firstwin)))) ? STATUS_HEIGHT : 0;
+  return (p_ls > 1 || (p_ls == 1 && (morewin || !one_window(firstwin, NULL)))) ? STATUS_HEIGHT : 0;
 }
 
 /// Return the minimal number of rows that is needed on the screen to display
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index f1579f5345..400c3611c7 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -1777,6 +1777,13 @@ describe('API/win', function()
     it('checks if splitting disallowed', function()
       command('split | autocmd WinEnter * ++once call nvim_open_win(0, 0, #{split: "right"})')
       matches("E242: Can't split a window while closing another$", pcall_err(command, 'quit'))
+      -- E242 is not needed for floats.
+      exec([[
+        split
+        autocmd WinEnter * ++once let g:win = nvim_open_win(0, 0, #{relative: "editor", row: 0, col: 0, width: 5, height: 5})
+        quit
+      ]])
+      eq('editor', eval('nvim_win_get_config(g:win).relative'))
 
       command('only | autocmd BufHidden * ++once call nvim_open_win(0, 0, #{split: "left"})')
       matches(
@@ -1803,6 +1810,38 @@ describe('API/win', function()
           .. '})'
       )
       command('new | quit')
+
+      -- Apply to opening floats too, as that can similarly create new views into a closing buffer.
+      -- For example, the following would open a float into an unloaded buffer:
+      exec([[
+        only
+        new
+        let g:buf = bufnr()
+        autocmd BufUnload * ++once call nvim_open_win(g:buf, 0, #{relative: "editor", width: 5, height: 5, row: 1, col: 1})
+        setlocal bufhidden=unload
+      ]])
+      matches('E1159: Cannot open a float when closing the buffer$', pcall_err(command, 'quit'))
+      eq(false, eval('nvim_buf_is_loaded(g:buf)'))
+      eq(0, eval('win_findbuf(g:buf)->len()'))
+
+      -- Only checking b_locked_split for the target buffer is insufficient, as naughty autocommands
+      -- can cause win_set_buf to remain in a closing curbuf:
+      exec([[
+        only
+        new
+        let g:buf = bufnr()
+        autocmd BufWipeout * ++once ++nested let g:buf2 = nvim_create_buf(1, 0)
+              \| execute 'autocmd BufLeave * ++once call nvim_buf_delete(g:buf2, #{force: 1})'
+              \| setlocal bufhidden=
+              \| call nvim_open_win(g:buf2, 1, #{relative: 'editor', width: 5, height: 5, col: 5, row: 5})
+        setlocal bufhidden=wipe
+      ]])
+      matches('E1159: Cannot open a float when closing the buffer$', pcall_err(command, 'quit'))
+      eq(false, eval('nvim_buf_is_loaded(g:buf)'))
+      eq(0, eval('win_findbuf(g:buf)->len()'))
+      -- BufLeave shouldn't run here (buf2 isn't deleted and remains hidden)
+      eq(true, eval('nvim_buf_is_loaded(g:buf2)'))
+      eq(0, eval('win_findbuf(g:buf2)->len()'))
     end)
 
     it('restores last known cursor position if BufWinEnter did not move it', function()
@@ -1931,6 +1970,80 @@ describe('API/win', function()
       )
     end)
 
+    it('no crash when closing the only non-float in other tabpage #31236', function()
+      local tp = api.nvim_get_current_tabpage()
+      local split_win = api.nvim_get_current_win()
+      local float_win = api.nvim_open_win(
+        0,
+        false,
+        { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+      )
+      command('tabnew')
+
+      api.nvim_win_close(split_win, false)
+      eq(false, api.nvim_win_is_valid(split_win))
+      eq(false, api.nvim_win_is_valid(float_win))
+      eq(false, api.nvim_tabpage_is_valid(tp))
+
+      tp = api.nvim_get_current_tabpage()
+      split_win = api.nvim_get_current_win()
+      local float_buf = api.nvim_create_buf(true, false)
+      float_win = api.nvim_open_win(
+        float_buf,
+        false,
+        { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+      )
+      -- Set these options to prevent the float from being automatically closed.
+      api.nvim_set_option_value('modified', true, { buf = float_buf })
+      api.nvim_set_option_value('bufhidden', 'wipe', { buf = float_buf })
+      command('tabnew')
+
+      matches(
+        'E5601: Cannot close window, only floating window would remain$',
+        pcall_err(api.nvim_win_close, split_win, false)
+      )
+      eq(true, api.nvim_win_is_valid(split_win))
+      eq(true, api.nvim_win_is_valid(float_win))
+      eq(true, api.nvim_tabpage_is_valid(tp))
+
+      api.nvim_set_current_win(float_win)
+      api.nvim_win_close(split_win, true) -- Force it this time.
+      eq(false, api.nvim_win_is_valid(split_win))
+      eq(false, api.nvim_win_is_valid(float_win))
+      eq(false, api.nvim_tabpage_is_valid(tp))
+
+      -- Ensure opening a float after the initial check (like in WinClosed) doesn't crash...
+      exec([[
+        tabnew
+        let g:tp = nvim_get_current_tabpage()
+        let g:win = win_getid()
+        tabprevious
+        autocmd! WinClosed * ++once call nvim_open_win(0, 0, #{win: g:win, relative: 'win', width: 5, height: 5, row: 5, col: 5})
+      ]])
+      matches(
+        'E5601: Cannot close window, only floating window would remain$',
+        pcall_err(command, 'call nvim_win_close(g:win, 0)')
+      )
+      eq(true, eval 'nvim_tabpage_is_valid(g:tp)')
+
+      exec([[
+        tabnew
+        let g:tp = nvim_get_current_tabpage()
+        let g:win = win_getid()
+        let g:buf = bufnr()
+        tabprevious
+        let s:buf2 = nvim_create_buf(0, 0)
+        call setbufvar(s:buf2, '&modified', 1)
+        call setbufvar(s:buf2, '&bufhidden', 'wipe')
+        autocmd! WinClosed * ++once call nvim_open_win(s:buf2, 0, #{win: g:win, relative: 'win', width: 5, height: 5, row: 5, col: 5})
+      ]])
+      matches(
+        'E5601: Cannot close window, only floating window would remain$',
+        pcall_err(command, 'call nvim_buf_delete(g:buf, #{force: 1})')
+      )
+      eq(true, eval 'nvim_tabpage_is_valid(g:tp)')
+    end)
+
     it('respects requested size for large splits', function()
       command('vsplit')
       local win = api.nvim_open_win(0, false, { win = -1, split = 'right', width = 38 })
@@ -1955,6 +2068,31 @@ describe('API/win', function()
       api.nvim_open_win(0, true, { split = 'below' })
       eq(11, api.nvim_win_get_height(0))
     end)
+
+    it('no leak when win_set_buf fails and window is closed immediately', function()
+      -- Following used to leak.
+      command('autocmd BufEnter * ++once quit! | throw 1337')
+      eq(
+        'Window was closed immediately',
+        pcall_err(
+          api.nvim_open_win,
+          api.nvim_create_buf(true, true),
+          true,
+          { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+        )
+      )
+      -- If the window wasn't closed, still set errors from win_set_buf.
+      command('autocmd BufEnter * ++once throw 1337')
+      eq(
+        'BufEnter Autocommands for "*": 1337',
+        pcall_err(
+          api.nvim_open_win,
+          api.nvim_create_buf(true, true),
+          true,
+          { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+        )
+      )
+    end)
   end)
 
   describe('set_config', function()
@@ -2005,11 +2143,42 @@ describe('API/win', function()
       eq('editor', api.nvim_win_get_config(win).relative)
     end)
 
-    it('throws error when attempting to move the last window', function()
+    it('throws error when attempting to move the last non-floating window', function()
       local err = pcall_err(api.nvim_win_set_config, 0, {
         vertical = false,
       })
-      eq('Cannot move last window', err)
+      eq('Cannot move last non-floating window', err)
+
+      local win1 = api.nvim_get_current_win()
+      command('tabnew')
+      eq(
+        'Cannot move last non-floating window',
+        pcall_err(api.nvim_win_set_config, 0, { win = win1, split = 'left' })
+      )
+      api.nvim_open_win(0, false, { relative = 'editor', width = 5, height = 5, row = 1, col = 1 })
+      eq(
+        'Cannot move last non-floating window',
+        pcall_err(api.nvim_win_set_config, 0, { win = win1, split = 'left' })
+      )
+
+      -- If it's no longer the last non-float, still an error if autocommands make it the last
+      -- non-float again before it's moved.
+      command('vsplit')
+      exec_lua(function()
+        vim.api.nvim_create_autocmd('WinEnter', {
+          once = true,
+          callback = function()
+            vim.api.nvim_win_set_config(
+              0,
+              { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+            )
+          end,
+        })
+      end)
+      eq(
+        'Cannot move last non-floating window',
+        pcall_err(api.nvim_win_set_config, 0, { win = win1, split = 'left' })
+      )
     end)
 
     it('passing retval of get_config results in no-op', function()
diff --git a/test/functional/autocmd/autocmd_spec.lua b/test/functional/autocmd/autocmd_spec.lua
index fdf07a2d39..b4317a3e50 100644
--- a/test/functional/autocmd/autocmd_spec.lua
+++ b/test/functional/autocmd/autocmd_spec.lua
@@ -17,7 +17,6 @@ local pcall_err = t.pcall_err
 local fn = n.fn
 local expect = n.expect
 local command = n.command
-local exc_exec = n.exc_exec
 local exec_lua = n.exec_lua
 local retry = t.retry
 local source = n.source
@@ -151,7 +150,10 @@ describe('autocmd', function()
       })
 
       command('autocmd BufLeave * bwipeout yy')
-      eq('Vim(edit):E143: Autocommands unexpectedly deleted new buffer yy', exc_exec('edit yy'))
+      eq(
+        'Vim(edit):E143: Autocommands unexpectedly deleted new buffer yy',
+        pcall_err(command, 'edit yy')
+      )
 
       expect([[
         start of test file xx
@@ -429,6 +431,23 @@ describe('autocmd', function()
     )
   end)
 
+  it('cannot close `aucmd_win` in non-current tabpage', function()
+    exec([[
+      file Xa
+      tabnew Xb
+      call setline(1, 'foo')
+      tabfirst
+      autocmd BufWriteCmd Xb tablast | bwipe! Xa
+    ]])
+    eq(
+      'BufWriteCmd Autocommands for "Xb": Vim(bwipeout):E813: Cannot close autocmd window',
+      pcall_err(command, 'wall')
+    )
+    -- Sanity check: :bwipe failing to close all windows into Xa should keep it loaded.
+    -- (So there's no risk of it being left unloaded inside a window)
+    eq(1, eval('bufloaded("Xa")'))
+  end)
+
   describe('closing last non-floating window in tab from `aucmd_win`', function()
     before_each(function()
       command('edit Xa.txt')
diff --git a/test/functional/autocmd/tabclose_spec.lua b/test/functional/autocmd/tabclose_spec.lua
index fde72cf4d7..3969f33cf1 100644
--- a/test/functional/autocmd/tabclose_spec.lua
+++ b/test/functional/autocmd/tabclose_spec.lua
@@ -2,8 +2,10 @@ local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 
 local clear, eq = n.clear, t.eq
-local api = n.api
 local command = n.command
+local eval = n.eval
+local exec = n.exec
+local exec_capture = n.exec_capture
 
 describe('TabClosed', function()
   before_each(clear)
@@ -16,11 +18,11 @@ describe('TabClosed', function()
         )
         repeat
           command('tabnew')
-        until api.nvim_eval('tabpagenr()') == 6 -- current tab is now 6
-        eq('tabclosed:6:6:5', api.nvim_exec('tabclose', true)) -- close last 6, current tab is now 5
-        eq('tabclosed:5:5:4', api.nvim_exec('close', true)) -- close last window on tab, closes tab
-        eq('tabclosed:2:2:3', api.nvim_exec('2tabclose', true)) -- close tab 2, current tab is now 3
-        eq('tabclosed:1:1:2\ntabclosed:1:1:1', api.nvim_exec('tabonly', true)) -- close tabs 1 and 2
+        until eval('tabpagenr()') == 6 -- current tab is now 6
+        eq('tabclosed:6:6:5', exec_capture('tabclose')) -- close last 6, current tab is now 5
+        eq('tabclosed:5:5:4', exec_capture('close')) -- close last window on tab, closes tab
+        eq('tabclosed:2:2:3', exec_capture('2tabclose')) -- close tab 2, current tab is now 3
+        eq('tabclosed:1:1:2\ntabclosed:1:1:1', exec_capture('tabonly')) -- close tabs 1 and 2
       end)
 
       it('is triggered when closing a window via bdelete from another tab', function()
@@ -30,9 +32,9 @@ describe('TabClosed', function()
         command('1tabedit Xtestfile')
         command('1tabedit Xtestfile')
         command('normal! 1gt')
-        eq({ 1, 3 }, api.nvim_eval('[tabpagenr(), tabpagenr("$")]'))
-        eq('tabclosed:2:2:1\ntabclosed:2:2:1', api.nvim_exec('bdelete Xtestfile', true))
-        eq({ 1, 1 }, api.nvim_eval('[tabpagenr(), tabpagenr("$")]'))
+        eq({ 1, 3 }, eval('[tabpagenr(), tabpagenr("$")]'))
+        eq('tabclosed:2:2:1\ntabclosed:2:2:1', exec_capture('bdelete Xtestfile'))
+        eq({ 1, 1 }, eval('[tabpagenr(), tabpagenr("$")]'))
       end)
 
       it('is triggered when closing a window via bdelete from current tab', function()
@@ -44,9 +46,39 @@ describe('TabClosed', function()
         command('1tabedit Xtestfile2')
 
         -- Only one tab is closed, and the alternate file is used for the other.
-        eq({ 2, 3 }, api.nvim_eval('[tabpagenr(), tabpagenr("$")]'))
-        eq('tabclosed:2:2:2', api.nvim_exec('bdelete Xtestfile2', true))
-        eq('Xtestfile1', api.nvim_eval('bufname("")'))
+        eq({ 2, 3 }, eval('[tabpagenr(), tabpagenr("$")]'))
+        eq('tabclosed:2:2:2', exec_capture('bdelete Xtestfile2'))
+        eq('Xtestfile1', eval('bufname("")'))
+      end)
+
+      it('triggers after tab page is properly freed', function()
+        exec([[
+          let s:tp = nvim_get_current_tabpage()
+          let g:buf = bufnr()
+
+          setlocal bufhidden=wipe
+          tabnew
+          au TabClosed * ++once let g:tp_valid = nvim_tabpage_is_valid(s:tp)
+                             \| let g:abuf = expand('<abuf>')
+
+          call nvim_buf_delete(g:buf, #{force: 1})
+        ]])
+        eq(false, eval('g:tp_valid'))
+        eq(false, eval('nvim_buf_is_valid(g:buf)'))
+        eq('', eval('g:abuf'))
+
+        exec([[
+          tabnew
+          let g:buf = bufnr()
+          let s:win = win_getid()
+
+          tabfirst
+          au TabClosed * ++once let g:abuf = expand('<abuf>')
+
+          call nvim_win_close(s:win, 1)
+        ]])
+        eq(true, eval('nvim_buf_is_valid(g:buf)'))
+        eq(eval('g:buf'), tonumber(eval('g:abuf')))
       end)
     end)
 
@@ -58,11 +90,11 @@ describe('TabClosed', function()
         command('au! TabClosed 2 echom "tabclosed:match"')
         repeat
           command('tabnew')
-        until api.nvim_eval('tabpagenr()') == 7 -- current tab is now 7
+        until eval('tabpagenr()') == 7 -- current tab is now 7
         -- sanity check, we shouldn't match on tabs with numbers other than 2
-        eq('tabclosed:7:7:6', api.nvim_exec('tabclose', true))
+        eq('tabclosed:7:7:6', exec_capture('tabclose'))
         -- close tab page 2, current tab is now 5
-        eq('tabclosed:2:2:5\ntabclosed:match', api.nvim_exec('2tabclose', true))
+        eq('tabclosed:2:2:5\ntabclosed:match', exec_capture('2tabclose'))
       end)
     end)
 
@@ -72,9 +104,9 @@ describe('TabClosed', function()
           'au! TabClosed * echom "tabclosed:".expand("<afile>").":".expand("<amatch>").":".tabpagenr()'
         )
         command('tabedit Xtestfile')
-        eq({ 2, 2 }, api.nvim_eval('[tabpagenr(), tabpagenr("$")]'))
-        eq('tabclosed:2:2:1', api.nvim_exec('close', true))
-        eq({ 1, 1 }, api.nvim_eval('[tabpagenr(), tabpagenr("$")]'))
+        eq({ 2, 2 }, eval('[tabpagenr(), tabpagenr("$")]'))
+        eq('tabclosed:2:2:1', exec_capture('close'))
+        eq({ 1, 1 }, eval('[tabpagenr(), tabpagenr("$")]'))
       end)
     end)
   end)
diff --git a/test/functional/editor/tabpage_spec.lua b/test/functional/editor/tabpage_spec.lua
index e829821a64..c8be39ea6e 100644
--- a/test/functional/editor/tabpage_spec.lua
+++ b/test/functional/editor/tabpage_spec.lua
@@ -147,4 +147,13 @@ describe('tabpage', function()
     command('tabs')
     assert_alive()
   end)
+
+  it('no crash if autocmd remains in tabpage of closing last window', function()
+    exec([[
+      tabnew
+      let s:win = win_getid()
+      autocmd TabLeave * ++once tablast | tabonly
+      quit
+    ]])
+  end)
 end)
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index c8492b0f3e..34d7d7be7a 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -1005,7 +1005,7 @@ describe('float window', function()
       assert_alive()
     end)
 
-    pending('does not crash if BufUnload makes it the only non-float in tabpage', function()
+    it('does not crash if BufUnload makes it the only non-float in tabpage', function()
       exec([[
         tabnew
         let g:buf = bufnr()
@@ -1017,10 +1017,60 @@ describe('float window', function()
               \ #{relative: 'editor', row: 5, col: 5, width: 5, height: 5})
         autocmd BufUnload * ++once exe g:buf .. 'bwipe!'
       ]])
-      command('close')
+      eq('Vim(close):E5601: Cannot close window, only floating window would remain', pcall_err(command, 'close'))
       assert_alive()
     end)
 
+    describe('does not crash if WinClosed makes it the only non-float', function()
+      before_each(function()
+        exec([[
+          let g:buf = bufnr()
+          new
+          setlocal bufhidden=wipe
+          autocmd WinClosed * ++once exe g:buf .. 'bwipe!'
+        ]])
+      end)
+
+      local opts = { relative = 'editor', row = 5, col = 5, width = 5, height = 5 }
+      local floatwin
+
+      describe('and there is a float window with the same buffer', function()
+        before_each(function()
+          floatwin = api.nvim_open_win(0, false, opts)
+        end)
+
+        it('with multiple tabpages', function()
+          command('tabnew | tabprev')
+          eq('Vim(close):E5601: Cannot close window, only floating window would remain', pcall_err(command, 'close'))
+          api.nvim_win_close(floatwin, true)
+          assert_alive()
+        end)
+
+        it('with only one tabpage', function()
+          command('close')
+          api.nvim_win_close(floatwin, true)
+          assert_alive()
+        end)
+      end)
+
+      describe('and there is a float with a different buffer', function()
+        before_each(function()
+          floatwin = api.nvim_open_win(api.nvim_create_buf(true, false), false, opts)
+        end)
+
+        it('with multiple tabpages', function()
+          command('tabnew | tabprev')
+          eq('Vim(close):E855: Autocommands caused command to abort', pcall_err(command, 'close'))
+          assert_alive()
+        end)
+
+        it('with only one tabpage', function()
+          eq('Vim(close):E855: Autocommands caused command to abort', pcall_err(command, 'close'))
+          assert_alive()
+        end)
+      end)
+    end)
+
     it('does not crash if WinClosed from floating window closes it', function()
       exec([[
         tabnew