patch 9.0.1726: incorrect heights in win_size_restore()

Problem: incorrect heights in win_size_restore()
Solution: avoid restoring incorrect heights in win_size_restore()

Changing 'showtabline' or 'cmdheight' in the cmdwin restores incorrect
window heights after closing the cmdwin.

This may produce a gap between the cmdline and the window above.

Solution: restore window sizes only if the number of lines available for windows
changed; subtract the rows of the tabline, cmdline and last window's statusline
from 'lines' (other statuslines don't matter).

closes: #12704

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>
diff --git a/src/testdir/test_cmdwin.vim b/src/testdir/test_cmdwin.vim
index e0b5833..5ba16db 100644
--- a/src/testdir/test_cmdwin.vim
+++ b/src/testdir/test_cmdwin.vim
@@ -440,5 +440,34 @@
   let &columns = columns
 endfunc
 
+func Test_cmdwin_restore_heights()
+  set showtabline=0 cmdheight=2 laststatus=0
+  call feedkeys("q::set cmdheight=1\<CR>:q\<CR>", 'ntx')
+  call assert_equal(&lines - 1, winheight(0))
+
+  set showtabline=2 cmdheight=3
+  call feedkeys("q::set showtabline=0\<CR>:q\<CR>", 'ntx')
+  call assert_equal(&lines - 3, winheight(0))
+
+  set cmdheight=1 laststatus=2
+  call feedkeys("q::set laststatus=0\<CR>:q\<CR>", 'ntx')
+  call assert_equal(&lines - 1, winheight(0))
+
+  set laststatus=2
+  call feedkeys("q::set laststatus=1\<CR>:q\<CR>", 'ntx')
+  call assert_equal(&lines - 1, winheight(0))
+
+  set laststatus=2
+  belowright vsplit
+  wincmd _
+  let restcmds = winrestcmd()
+  call feedkeys("q::set laststatus=1\<CR>:q\<CR>", 'ntx')
+  " 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.
+  call assert_equal(restcmds, winrestcmd())
+
+  set cmdheight& showtabline& laststatus&
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab