vim-patch:9.1.0059: No event triggered before creating a window

Problem:  No event is triggered before creating a window.
          (Sergey Vlasov)
Solution: Add the WinNewPre event (Sergey Vlasov)

fixes: vim/vim#10635
closes: vim/vim#12761

1f47db75fd

Not sure if this should be triggered before creating a floating window,
as its use case is related to window layout.

Co-authored-by: Sergey Vlasov <sergey@vlasov.me>

runtime/doc/autocmd.txt

@@ -1232,6 +1232,18 @@ WinLeave			Before leaving a window.  If the window to be
 				WinLeave autocommands (but not for ":new").
 				Not used for ":qa" or ":q" when exiting Vim.
 				Before WinClosed.
+							*WinNewPre*
+WinNewPre			Before creating a new window. Triggered
+				before commands that modify window layout by
+				creating a split or new tab page. Not done for
+				the first window, when Vim has just started.
+				It is not allowed to modify window layout
+				while executing commands for the WinNewPre
+				event.
+				Most useful to store current window layout
+				and compare it with the new layout after the
+				Window has been created.
+
 							*WinNew*
 WinNew				When a new window was created.  Not done for
 				the first window, when Vim has just started.

runtime/doc/options.txt

@@ -2580,7 +2580,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 		|VimResized|,
 		|VimResume|,
 		|VimSuspend|,
-		|WinNew|
+		|WinNew|,
+		|WinNewPre|
 
 				*'expandtab'* *'et'* *'noexpandtab'* *'noet'*
 'expandtab' 'et'	boolean	(default off)

runtime/lua/vim/_meta/api_keysets.lua

@@ -218,6 +218,7 @@ error('Cannot require a meta file')
 --- |'WinEnter'
 --- |'WinLeave'
 --- |'WinNew'
+--- |'WinNewPre'
 --- |'WinResized'
 --- |'WinScrolled'
 

runtime/lua/vim/_meta/options.lua

@@ -2258,7 +2258,8 @@ vim.go.ei = vim.go.eventignore
 --- 	`VimResized`,
 --- 	`VimResume`,
 --- 	`VimSuspend`,
---- 	`WinNew`
+--- 	`WinNew`,
+--- 	`WinNewPre`
 ---
 --- @type string
 vim.o.eventignorewin = ""

src/nvim/auevents.lua

@@ -138,7 +138,8 @@ return {
     WinClosed = true, -- after closing a window
     WinEnter = true, -- after entering a window
     WinLeave = true, -- before leaving a window
-    WinNew = false, -- when entering a new window
+    WinNewPre = false, -- before creating a new window
+    WinNew = false, -- after creating a new window
     WinResized = true, -- after a window was resized
     WinScrolled = true, -- after a window was scrolled or resized
   },

src/nvim/window.c

@@ -1127,6 +1127,8 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
     return NULL;
   }
 
+  trigger_winnewpre();
+
   win_T *oldwin;
   if (flags & WSP_TOP) {
     oldwin = firstwin;
@@ -3069,6 +3071,13 @@ int win_close(win_T *win, bool free_buf, bool force)
   return OK;
 }
 
+static void trigger_winnewpre(void)
+{
+  window_layout_lock();
+  apply_autocmds(EVENT_WINNEWPRE, NULL, NULL, false, NULL);
+  window_layout_unlock();
+}
+
 static void do_autocmd_winclosed(win_T *win)
   FUNC_ATTR_NONNULL_ALL
 {
@@ -4380,6 +4389,8 @@ int win_new_tabpage(int after, char *filename)
 
   curtab = newtp;
 
+  trigger_winnewpre();
+
   // Create a new empty window.
   if (win_alloc_firstwin(old_curtab->tp_curwin) == OK) {
     // Make the new Tab page the new topframe.

test/old/testdir/test_autocmd.vim

@@ -277,6 +277,7 @@ func Test_win_tab_autocmd()
   let g:record = []
 
   augroup testing
+    au WinNewPre * call add(g:record, 'WinNewPre')
     au WinNew * call add(g:record, 'WinNew')
     au WinClosed * call add(g:record, 'WinClosed')
     au WinEnter * call add(g:record, 'WinEnter')
@@ -293,8 +294,8 @@ func Test_win_tab_autocmd()
   close
 
   call assert_equal([
-	\ 'WinLeave', 'WinNew', 'WinEnter',
-	\ 'WinLeave', 'TabLeave', 'WinNew', 'WinEnter', 'TabNew', 'TabEnter',
+	\ 'WinNewPre', 'WinLeave', 'WinNew', 'WinEnter',
+	\ 'WinLeave', 'TabLeave', 'WinNewPre', 'WinNew', 'WinEnter', 'TabNew', 'TabEnter',
 	\ 'WinLeave', 'TabLeave', 'WinClosed', 'TabClosed', 'WinEnter', 'TabEnter',
 	\ 'WinLeave', 'WinClosed', 'WinEnter'
 	\ ], g:record)
@@ -305,17 +306,96 @@ func Test_win_tab_autocmd()
   bwipe somefile
 
   call assert_equal([
-	\ 'WinLeave', 'TabLeave', 'WinNew', 'WinEnter', 'TabNew', 'TabEnter',
+	\ 'WinLeave', 'TabLeave', 'WinNewPre', 'WinNew', 'WinEnter', 'TabNew', 'TabEnter',
 	\ 'WinLeave', 'TabLeave', 'WinEnter', 'TabEnter',
 	\ 'WinClosed', 'TabClosed'
 	\ ], g:record)
 
+  let g:record = []
+  copen
+  help
+  tabnext
+  vnew
+
+  call assert_equal([
+	\ 'WinNewPre', 'WinLeave', 'WinNew', 'WinEnter',
+	\ 'WinNewPre', 'WinLeave', 'WinNew', 'WinEnter',
+	\ 'WinNewPre', 'WinLeave', 'WinNew', 'WinEnter'
+	\ ], g:record)
+
   augroup testing
     au!
   augroup END
   unlet g:record
 endfunc
 
+func Test_WinNewPre()
+  " Test that the old window layout can be accessed before a new window is created.
+  let g:layouts_pre = []
+  let g:layouts_post = []
+  augroup testing
+    au WinNewPre * call add(g:layouts_pre, winlayout())
+    au WinNew * call add(g:layouts_post, winlayout())
+  augroup END
+  split
+  call assert_notequal(g:layouts_pre[0], g:layouts_post[0])
+  split
+  call assert_equal(g:layouts_pre[1], g:layouts_post[0])
+  call assert_notequal(g:layouts_pre[1], g:layouts_post[1])
+  tabnew
+  call assert_notequal(g:layouts_pre[2], g:layouts_post[1])
+  call assert_notequal(g:layouts_pre[2], g:layouts_post[2])
+  augroup testing
+    au!
+  augroup END
+  unlet g:layouts_pre
+  unlet g:layouts_post
+
+  " Test modifying window layout during WinNewPre throws.
+  let g:caught = 0
+  augroup testing
+    au!
+    au WinNewPre * split
+  augroup END
+  try
+    vnew
+  catch
+    let g:caught += 1
+  endtry
+  augroup testing
+    au!
+    au WinNewPre * tabnew
+  augroup END
+  try
+    vnew
+  catch
+    let g:caught += 1
+  endtry
+  augroup testing
+    au!
+    au WinNewPre * close
+  augroup END
+  try
+    vnew
+  catch
+    let g:caught += 1
+  endtry
+  augroup testing
+    au!
+    au WinNewPre * tabclose
+  augroup END
+  try
+    vnew
+  catch
+    let g:caught += 1
+  endtry
+  call assert_equal(4, g:caught)
+  augroup testing
+    au!
+  augroup END
+  unlet g:caught
+endfunc
+
 func Test_WinResized()
   CheckRunVimInTerminal