fix(grid): assert crash during extreme resize layouts #36847

Problem:
When the terminal shrinks, Neovim tries to recompute the layout and
adjust every window so that the total width matches the new Columns. But
in the scenario of numerous windows split horizontally, there is
not enough space for all of them. The frame width (topframe->fr_width) never
reaches the new Columns value, the logic tries to redistribute this negative
space across child frames, but it triggers the assert.

Solution:
Skip this case in `win_update`.

src/nvim/drawscreen.c

@@ -1367,6 +1367,12 @@ static void draw_sep_connectors_win(win_T *wp)
 /// bot: from bot_start to last row (when scrolled up)
 static void win_update(win_T *wp)
 {
+  // Return early when the windows would overflow the shrunk terminal window
+  // avoiding invalid drawing an assert failure
+  if (wp->w_grid.target == &default_grid && wp->w_wincol >= Columns) {
+    return;
+  }
+
   int top_end = 0;              // Below last row of the top area that needs
                                 // updating.  0 when no top area updating.
   int mid_start = 999;          // first row of the mid area that needs

test/functional/ui/screen_basic_spec.lua

@@ -2,7 +2,7 @@ local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 local Screen = require('test.functional.ui.screen')
 
-local set_session, clear = n.set_session, n.clear
+local set_session, clear, assert_alive = n.set_session, n.clear, n.assert_alive
 local feed, command = n.feed, n.command
 local exec = n.exec
 local insert = n.insert
@@ -646,6 +646,20 @@ local function screen_tests(linegrid)
       ]])
     end)
 
+    it('does not crash when windows fill the screen #33883', function()
+      screen:try_resize(80, 20)
+      while true do
+        local ok = pcall(command, 'wincmd v')
+        if not ok then
+          break
+        end
+      end
+
+      screen:try_resize(60, 20)
+
+      assert_alive()
+    end)
+
     it('clamps &cmdheight for current tabpage', function()
       command('set cmdheight=10 laststatus=2')
       screen:expect([[