fix(float): :bufdelete in floatwin may change focus #39858

Problem:
(Followup to 54f22a8f01c0feb27a531b52aedf5cdbd5e51b24.)
Deleting another buffer from a floatwin could move focus into the holder
window and fire BufEnter for the buffer being deleted.

Solution:
Use switch_win_noblock() instead of buf_jump_open_win() before
recursing into do_buffer_ext().

src/nvim/buffer.c

@@ -46,6 +46,7 @@
 #include "nvim/errors.h"
 #include "nvim/eval/typval.h"
 #include "nvim/eval/vars.h"
+#include "nvim/eval/window.h"
 #include "nvim/ex_cmds.h"
 #include "nvim/ex_cmds2.h"
 #include "nvim/ex_cmds_defs.h"
@@ -1462,11 +1463,21 @@ static int do_buffer_ext(int action, int start, int dir, int count, int flags)
       }
 
       close_windows(buf, false);
-      // close_windows keeps the last non-float window. If it still
-      // shows buf, jump there and retry so curbuf gets replaced.
-      if (bufref_valid(&bufref) && buf->b_nwindows > 0) {
-        win_T *holder = buf_jump_open_win(buf);
-        if (holder != NULL && !holder->w_floating) {
+      // close_windows() refuses to close curtab's last non-float window.
+      // If it still shows buf, retry the delete from there.
+      win_T *holder = close_windows(buf, false);
+      if (buf != curbuf && bufref_valid(&bufref) && holder != NULL) {
+        if (curwin->w_floating) {
+          // swap into holder without entering it: caller keeps focus,
+          // BufEnter doesn't fire for the deleted buffer.
+          switchwin_T switchwin;
+          const int rv = switch_win_noblock(&switchwin, holder, curtab, true);
+          assert(rv == OK);
+          (void)rv;
+          do_buffer_ext(action, start, dir, count, flags);
+          restore_win_noblock(&switchwin, true);
+        } else {
+          buf_jump_open_win(buf);
           return do_buffer_ext(action, start, dir, count, flags);
         }
       }

src/nvim/window.c

@@ -2593,7 +2593,8 @@ void curwin_init(void)
 /// Closes all windows for buffer `buf` unless there is only one non-floating window.
 ///
 /// @param keep_curwin  don't close `curwin`
-void close_windows(buf_T *buf, bool keep_curwin)
+/// @return the kept holder window, or NULL.
+win_T *close_windows(buf_T *buf, bool keep_curwin)
 {
   RedrawingDisabled++;
 
@@ -2646,6 +2647,12 @@ void close_windows(buf_T *buf, bool keep_curwin)
 
 theend:
   RedrawingDisabled--;
+
+  // If buf is still shown in the last non-float window, let the caller retry there.
+  if (firstwin->w_buffer == buf && !firstwin->w_floating) {
+    return firstwin;
+  }
+  return NULL;
 }
 
 /// Check if "win" is the last non-floating window that exists.

test/functional/ui/float_spec.lua

@@ -764,6 +764,7 @@ describe('float window', function()
         it('if called from floating window with another buffer', function()
           api.nvim_set_current_win(other_buf_float)
           api.nvim_buf_delete(old_buf, { force = true })
+          eq(other_buf_float, curwin())
         end)
       end)
       describe('creates an empty buffer when there is only one listed buffer', function()
@@ -776,13 +777,11 @@ describe('float window', function()
           command('set nobuflisted')
           api.nvim_set_current_win(old_win)
         end)
-        after_each(function()
-          expect('')
-          eq(2, #api.nvim_list_wins())
-        end)
         it('if called from non-floating window', function()
           api.nvim_buf_delete(old_buf, { force = true })
           eq(old_win, curwin())
+          expect('')
+          eq(2, #api.nvim_list_wins())
         end)
         it('if called from floating window with the same buffer', function()
           api.nvim_set_current_win(same_buf_float)
@@ -792,12 +791,36 @@ describe('float window', function()
           eq(same_buf_float, eval('g:win_leave'))
           eq(old_win, eval('g:win_enter'))
           eq(old_win, curwin())
+          expect('')
+          eq(2, #api.nvim_list_wins())
         end)
         it('if called from floating window with an unlisted buffer', function()
           api.nvim_set_current_win(unlisted_buf_float)
           api.nvim_buf_delete(old_buf, { force = true })
+          eq(unlisted_buf_float, curwin())
+          expect('unlisted')
+          eq('', fn.bufname(api.nvim_win_get_buf(old_win)))
+          eq(false, api.nvim_buf_is_valid(old_buf))
+          eq(2, #api.nvim_list_wins())
         end)
       end)
+      it('keeps focus in the floating window #39800', function()
+        api.nvim_open_win(old_buf, false, float_opts)
+        local other_float = api.nvim_open_win(api.nvim_create_buf(true, false), true, float_opts)
+        api.nvim_buf_delete(old_buf, { force = true })
+        eq(other_float, curwin())
+      end)
+
+      it('does not trigger BufEnter for deleted buffer #39800', function()
+        api.nvim_open_win(old_buf, false, float_opts)
+        api.nvim_open_win(api.nvim_create_buf(true, false), true, float_opts)
+        command('let g:abufs = []')
+        command('autocmd BufEnter * call add(g:abufs, +expand("<abuf>"))')
+        api.nvim_buf_delete(old_buf, { force = true })
+        for _, b in ipairs(eval('g:abufs')) do
+          neq(old_buf, b)
+        end
+      end)
     end)
     describe('with splits, deleting the last listed buffer creates an empty buffer', function()
       describe('when a non-floating window has an unlisted buffer', function()