screen: initialize screen properly with early `set display-=msgsep`

Currently `nvim -u NORC --cmd "set display-=msgsep"` will still allocate the
message grid and remove it just afterwards. While inefficient, we must
make sure update_screen() re-validates the default_grid completely when
this happens.

Fix some invalid logic: don't reallocate msg_grid on resize when the grid is not
used.

Elide a too early ui_flush() on startup, which caused an invalid cursor
position to be used.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 8f415a8ed5..b9469686b5 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -404,7 +404,7 @@ int update_screen(int type)
     default_grid.valid = true;
   }
 
-  if (type == NOT_VALID && msg_dothrottle()) {
+  if (type == NOT_VALID && (msg_dothrottle() || msg_grid.chars)) {
     grid_fill(&default_grid, Rows-p_ch, Rows, 0, Columns, ' ', ' ', 0);
   }
 
@@ -7288,9 +7288,9 @@ void screen_resize(int width, int height)
         }
       }
     }
+    ui_flush();
   }
-  ui_flush();
-  --busy;
+  busy--;
 }
 
 /// Check if the new Nvim application "shell" dimensions are valid.