fix(ui): make window resize commands manage cmdheight

Previously, the window resize commands did not resize the value of `cmdheight` when they caused a change in the topframe height, leaving a gap between the end of topframe and the start of the command line, this commit fixes that by making window resize commands automatically change the value of cmdheight if the resize affects the height of topframe.

src/nvim/window.c

@@ -5412,11 +5412,17 @@ void win_setheight_win(int height, win_T *win)
  // line, clear it.
  if (full_screen && msg_scrolled == 0 && row < cmdline_row) {
  grid_fill(&default_grid, row, cmdline_row, 0, Columns, ' ', ' ', 0);
+ if (msg_grid.chars) {
+ clear_cmdline = true;
+ }
  }
  cmdline_row = row;
+ p_ch = MAX(Rows - cmdline_row, 1);
+ curtab->tp_ch_used = p_ch;
  msg_row = row;
  msg_col = 0;
  redraw_all_later(NOT_VALID);
+ showmode();
  }
 }
 
@@ -5452,7 +5458,9 @@ static void frame_setheight(frame_T *curfrp, int height)
  if (curfrp->fr_parent == NULL) {
  // topframe: can only change the command line
  if (height > ROWS_AVAIL) {
- height = ROWS_AVAIL;
+ // If height is greater than the available space, try to create space for the frame by
+ // reducing 'cmdheight' if possible, while making sure `cmdheight` doesn't go below 1.
+ height = MIN(ROWS_AVAIL + (p_ch - 1), height);
  }
  if (height > 0) {
  frame_new_height(curfrp, height, false, false);

test/functional/legacy/cmdline_spec.lua

@@ -33,8 +33,8 @@ describe('cmdline', function()
  ~ |
  ~ |
  ~ |
- 6 |
- 7 |
+   |
+   |
  |
  ]])
  end)