fix(move): check the correct buffer

src/nvim/move.c

@@ -247,7 +247,7 @@ void update_topline(win_T *wp)
   int old_topfill = wp->w_topfill;
 
   // If the buffer is empty, always set topline to 1.
-  if (buf_is_empty(curbuf)) {             // special case - file is empty
+  if (buf_is_empty(wp->w_buffer)) {  // special case - file is empty
     if (wp->w_topline != 1) {
       redraw_later(wp, UPD_NOT_VALID);
     }

test/functional/ui/inccommand_spec.lua

@@ -3095,6 +3095,36 @@ it(':substitute with inccommand works properly if undo is not synced #20029', fu
     baz]])
 end)
 
+it(':substitute with inccommand does not unexpectedly change viewport #25697', function()
+  clear()
+  local screen = Screen.new(45, 5)
+  common_setup(screen, 'nosplit', long_multiline_text)
+  command('vnew | tabnew | tabclose')
+  screen:expect([[
+    ^                      │£ m n                 |
+    {15:~                     }│t œ ¥                 |
+    {15:~                     }│                      |
+    {11:[No Name]              }{10:[No Name] [+]         }|
+                                                 |
+  ]])
+  feed(':s/')
+  screen:expect([[
+                          │£ m n                 |
+    {15:~                     }│t œ ¥                 |
+    {15:~                     }│                      |
+    {11:[No Name]              }{10:[No Name] [+]         }|
+    :s/^                                          |
+  ]])
+  feed('<Esc>')
+  screen:expect([[
+    ^                      │£ m n                 |
+    {15:~                     }│t œ ¥                 |
+    {15:~                     }│                      |
+    {11:[No Name]              }{10:[No Name] [+]         }|
+                                                 |
+  ]])
+end)
+
 it('long :%s/ with inccommand does not collapse cmdline', function()
   clear()
   local screen = Screen.new(10,5)