When resizing the window, NvimViewport frames get dropped.

[PolyMeilex]

When resizing the window sometimes frames get randomly dropped.

Here are 3 frames from such a resize:

1	2	3

From a short debugging session it turns out that items of a line are not yet present, so there is some kind of race in there, cells are already updated, and I can get chars assigned to them, but items slice is empty, this results in empty snapshot.

• OS: Linux Ubuntu
• Neovim version: v0.8.3
• Neovim-Gtk build version: master 03a42d3226eb152e7b2c00064d3868310a221d01