fix(window): prevent win_size_restore from changing cmdheight

Currently it only skips if `Rows` changed, but it's possible for the height of
the usable area for windows to change (e.g: via `&ch`, `&stal` or `&ls`), which
can cause the value of `&cmdheight` to change when the sizes are restored.

This is a Vim bug, so I've submitted a PR there too. No telling when it'll be
merged though, given the current lack of activity there.

`ROWS_AVAIL` is convenient here, but also subtracts the `global_stl_height()`.
Not ideal, as we also care about the height of the last statusline for other
values of `&ls`. Meh.

Introduce `last_stl_height` for getting the height of the last statusline and
use it in `win_size_save/restore` and `last_status` (means
`last_status_rec`'s `statusline` argument will now be true if `&ls` is 3,
but that does not change the behaviour).

Also corrects the logic in `comp_col` to not assume there's a last statusline
if `&ls` is 1 and the last window is floating.

src/nvim/drawscreen.c

@@ -1137,7 +1137,7 @@ static void recording_mode(int attr)
 /// of 'ru_col'.
 void comp_col(void)
 {
-  int last_has_status = (p_ls > 1 || (p_ls == 1 && !ONE_WINDOW));
+  bool last_has_status = last_stl_height(false) > 0;
 
   sc_col = 0;
   ru_col = 0;

src/nvim/window.c

@@ -5712,8 +5712,9 @@ void win_size_save(garray_T *gap)
 {
   ga_init(gap, (int)sizeof(int), 1);
   ga_grow(gap, win_count() * 2 + 1);
-  // first entry is value of 'lines'
+  // first entry is the total lines available for windows
-  ((int *)gap->ga_data)[gap->ga_len++] = Rows;
+  ((int *)gap->ga_data)[gap->ga_len++] =
+    (int)ROWS_AVAIL + global_stl_height() - last_stl_height(false);
 
   FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
     ((int *)gap->ga_data)[gap->ga_len++] =
@@ -5723,13 +5724,14 @@ void win_size_save(garray_T *gap)
 }
 
 // Restore window sizes, but only if the number of windows is still the same
-// and 'lines' didn't change.
+// and total lines available for windows didn't change.
 // Does not free the growarray.
 void win_size_restore(garray_T *gap)
   FUNC_ATTR_NONNULL_ALL
 {
   if (win_count() * 2 + 1 == gap->ga_len
-      && ((int *)gap->ga_data)[0] == Rows) {
+      && ((int *)gap->ga_data)[0] ==
+      (int)ROWS_AVAIL + global_stl_height() - last_stl_height(false)) {
     // The order matters, because frames contain other frames, but it's
     // difficult to get right. The easy way out is to do it twice.
     for (int j = 0; j < 2; j++) {
@@ -6993,8 +6995,7 @@ char *file_name_in_line(char *line, int col, int options, int count, char *rel_f
 void last_status(bool morewin)
 {
   // Don't make a difference between horizontal or vertical split.
-  last_status_rec(topframe, (p_ls == 2 || (p_ls == 1 && (morewin || !one_nonfloat()))),
+  last_status_rec(topframe, last_stl_height(morewin) > 0, global_stl_height() > 0);
-                  global_stl_height() > 0);
 }
 
 // Remove status line from window, replacing it with a horizontal separator if needed.
@@ -7193,6 +7194,14 @@ int global_stl_height(void)
   return (p_ls == 3) ? STATUS_HEIGHT : 0;
 }
 
+/// Return the height of the last window's statusline, or the global statusline if set.
+///
+/// @param morewin  pretend there are two or more windows if true.
+int last_stl_height(bool morewin)
+{
+  return (p_ls > 1 || (p_ls == 1 && (!one_nonfloat() || morewin))) ? STATUS_HEIGHT : 0;
+}
+
 /// Return the minimal number of rows that is needed on the screen to display
 /// the current number of windows.
 int min_rows(void)

test/functional/ui/cmdline_spec.lua

@@ -984,14 +984,46 @@ it('tabline is not redrawn in Ex mode #24122', function()
 end)
 
 describe("cmdline height", function()
+  before_each(clear)
+
   it("does not crash resized screen #14263", function()
-    clear()
     local screen = Screen.new(25, 10)
     screen:attach()
     command('set cmdheight=9999')
     screen:try_resize(25, 5)
     assert_alive()
   end)
+
+  it('unchanged when trying to restore window sizes', function()
+    command('set showtabline=0 cmdheight=2 laststatus=0')
+    feed('q:')  -- Closing cmdwin tries to restore sizes
+    command('set cmdheight=1 | quit')
+    eq(1, eval('&cmdheight'))
+
+    command('set showtabline=2 cmdheight=3')
+    feed('q:')
+    command('set showtabline=0 | quit')
+    eq(3, eval('&cmdheight'))
+
+    command('set cmdheight=1 laststatus=2')
+    feed('q:')
+    command('set laststatus=0 | quit')
+    eq(1, eval('&cmdheight'))
+
+    command('set laststatus=2')
+    feed('q:')
+    command('set laststatus=1 | quit')
+    eq(1, eval('&cmdheight'))
+
+    command('set laststatus=2 | belowright vsplit | wincmd _')
+    local restcmds = eval('winrestcmd()')
+    feed('q:')
+    command('set laststatus=1 | quit')
+    -- As we have 2 windows, &ls = 1 should still have a statusline on the last
+    -- window. As such, the number of available rows hasn't changed and the window
+    -- sizes should be restored.
+    eq(restcmds, eval('winrestcmd()'))
+  end)
 end)
 
 describe('cmdheight=0', function()