fix(api): merge split window config only on success

Problem: nvim_win_set_config may merge configs despite failing to configure a
split, and without applying necessary side-effects (like setting style=minimal
options). Plus, autocommands may apply a different config after the merge,
causing side-effects to apply for an outdated config.

Solution: merge configs last, only on success. Include fields only relevant to
splits. Properly set _cmdline_offset for splits.

Maybe better to disallow _cmdline_offset for splits instead, as the pum is
relative to cmdline_row anyway? (I didn't want to change behaviour too much)

Also use expect_unchanged in an unrelated test to quash a warning.

src/nvim/api/win_config.c

@@ -431,7 +431,6 @@ static bool win_config_split(win_T *win, Dict(win_config) *config, WinConfig *fc
       fconfig->split = (old_split == kWinSplitBelow || p_sb) ? kWinSplitBelow : kWinSplitAbove;
     }
   }
-  merge_win_config(&win->w_config, *fconfig);
 
   // If there's no "vertical" or "split" set, or if "split" is unchanged, then we can just change
   // the size of the window.
@@ -605,6 +604,14 @@ resize:
   if (HAS_KEY_X(config, height)) {
     win_setheight_win(fconfig->height, win);
   }
+
+  // Merge configs now. If previously a float, clear fields irrelevant to splits that `fconfig` may
+  // have shallowly copied; don't free them as win_split_ins handled that. If already a split,
+  // clearing isn't needed, as parse_win_config shouldn't allow setting irrelevant fields.
+  if (!was_split) {
+    clear_float_config(fconfig, false);
+  }
+  merge_win_config(&win->w_config, *fconfig);
   return true;
 #undef HAS_KEY_X
 }
@@ -662,11 +669,9 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     win_config_float(win, fconfig);
   }
 
-  if (HAS_KEY_X(config, style)) {
-    if (fconfig.style == kWinStyleMinimal) {
-      win_set_minimal_style(win);
-      didset_window_options(win, true);
-    }
+  if (HAS_KEY_X(config, style) && fconfig.style == kWinStyleMinimal) {
+    win_set_minimal_style(win);
+    didset_window_options(win, true);
   }
   if (fconfig._cmdline_offset < INT_MAX) {
     cmdline_win = win;

src/nvim/window.c

@@ -864,6 +864,22 @@ void merge_win_config(WinConfig *dst, const WinConfig src)
   *dst = src;
 }
 
+/// Clear fields in `fconfig` that are only used for floating windows.
+/// Also clears fields unused after configure time, like width/height.
+void clear_float_config(WinConfig *fconfig, bool free_fields)
+  FUNC_ATTR_NONNULL_ALL
+{
+  WinStyle saved_style = fconfig->style;
+  int saved_cmdline_offset = fconfig->_cmdline_offset;
+  if (free_fields) {
+    merge_win_config(fconfig, WIN_CONFIG_INIT);
+  } else {
+    *fconfig = WIN_CONFIG_INIT;
+  }
+  fconfig->style = saved_style;
+  fconfig->_cmdline_offset = saved_cmdline_offset;
+}
+
 void ui_ext_win_position(win_T *wp, bool validate)
 {
   wp->w_pos_changed = false;
@@ -1364,7 +1380,6 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
     // make the contents of the new window the same as the current one
     win_init(wp, curwin, flags);
   } else if (wp->w_floating) {
-    WinStyle saved_style = wp->w_config.style;
     ui_comp_remove_grid(&wp->w_grid_alloc);
     if (ui_has(kUIMultigrid)) {
       wp->w_pos_changed = true;
@@ -1387,10 +1402,8 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
     new_frame(wp);
 
     // non-floating window doesn't store float config or have a border.
-    merge_win_config(&wp->w_config, WIN_CONFIG_INIT);
+    clear_float_config(&wp->w_config, true);
     CLEAR_FIELD(wp->w_border_adj);
-    // Restore WinConfig style. #37067
-    wp->w_config.style = saved_style;
   }
 
   // Going to reorganize frames now, make sure they're flat.

test/functional/api/window_spec.lua

@@ -3553,5 +3553,41 @@ describe('API/win', function()
       eq('', api.nvim_get_option_value('colorcolumn', { win = win }))
       eq('', api.nvim_get_option_value('statuscolumn', { win = win }))
     end)
+
+    it('merges configs only after successfully configuring split', function()
+      local win = api.nvim_open_win(0, true, {
+        relative = 'editor',
+        width = 10,
+        height = 10,
+        row = 5,
+        col = 5,
+      })
+      local cfg = api.nvim_win_get_config(win)
+      eq('', cfg.style)
+      command('set cursorline | tabnew')
+      local tp2_win = api.nvim_get_current_win()
+      command('tabfirst | autocmd WinEnter * ++once wincmd p')
+      eq(
+        'Failed to switch away from window 1001',
+        pcall_err(
+          api.nvim_win_set_config,
+          win,
+          { split = 'below', win = tp2_win, style = 'minimal' }
+        )
+      )
+      eq(cfg, api.nvim_win_get_config(win))
+      eq(true, api.nvim_get_option_value('cursorline', { win = win }))
+
+      exec([[
+        autocmd WinLeave * ++once let g:style_before = nvim_win_get_config(0).style
+                               \| let g:cul_before = &cursorline
+                               \| call nvim_win_set_config(0, #{style: ""})
+      ]])
+      api.nvim_win_set_config(win, { split = 'below', win = tp2_win, style = 'minimal' })
+      eq('', eval('g:style_before'))
+      eq(1, eval('g:cul_before'))
+      eq('minimal', api.nvim_win_get_config(win).style)
+      eq(false, api.nvim_get_option_value('cursorline', { win = win }))
+    end)
   end)
 end)

test/functional/lua/ui_event_spec.lua

@@ -390,6 +390,12 @@ describe('vim.ui_attach', function()
       9              bufname(       {12: }         |
       Excommand:call bufadd^(                  |
     ]])
+    -- _cmdline_offset remains set after being turned into a split.
+    exec_lua(function()
+      vim.fn.win_execute(_G.win, 'wincmd J')
+    end)
+    feed('<Tab>') -- Was a signed int overflow; offset was INT_MAX despite cmdline_win being set.
+    eq(9, exec_lua('return vim.api.nvim_win_get_config(_G.win)._cmdline_offset'))
     -- No crash after _cmdline_offset window is closed #35584.
     exec_lua(function()
       vim.ui_detach(_G.ns)

test/functional/ui/float_spec.lua

@@ -8942,17 +8942,7 @@ describe('float window', function()
           unchanged = true,
         }
       else
-        screen:expect([[
-          Ut enim ad minim veniam, quis nostrud             |
-          exercitation ullamco laboris nisi ut aliquip ex   |
-          ea co{2:test}{3:o consequat}. Duis aute irure dolor in    |
-          repre{3:henderit in vol}uptate velit esse cillum      |
-          dolor{2:popup}{3:fugi}{2:text}{3:ul}la pariatur. Excepteur sint   |
-          occaecat cupidatat non proident, sunt in culpa    |
-          qui officia deserunt mollit anim id est           |
-          laborum^.                                          |
-                                                            |
-        ]])
+        screen:expect_unchanged()
       end
       api.nvim_buf_set_lines(buf, 0, -1, true, test_data)