fix(extui): "box" window width calculated on last line (#33881)

Problem:  The "box" window width is calculated on the last line when
          applying "last" virt_text. There may be longer lines
          part of the same message.
Solution: Don't decrease box width if the calculated width is smaller.
          Minor unrelated changes.

Co-authored-by: Eike <eike.rackwitz@mail.de>

runtime/lua/vim/_extui.lua

@@ -96,6 +96,7 @@ function M.enable(opts)
   end
 
   if vim.v.vim_did_enter == 1 then
+    ext.tab_check_wins()
     check_opt('cmdheight', vim.o.cmdheight)
     check_opt('termguicolors', vim.o.termguicolors)
   end