Terminal goes blank sometimes, loadCell exception in console

[Tyriar]

[Tyriar]

https://github.com/Tyriar/xterm.js/blob/4c0c2559c33a02d147daedf80a45d82d6f543406/addons/addon-webgl/src/WebglRenderer.ts#L422

[Tyriar]

The terminal comes back to life if you make it small enough:

But dies again depending on the width:

[Tyriar]

🐛 row and ydisp are -1:

[Tyriar]

This is probably caused by bounds not being handled correctly when the new special conpty reflow logic was added xtermjs/xterm.js#5321

[Tyriar]

There's a related problem that goes away when I revert xtermjs/xterm.js#5321:

1. Load xterm.js demo
2. cat README.md
3. Resize to 50x30
4. Resize to 60x30, 🐛 the buffer is just broken. It goes from 400+ lines to only the first 40 or so lines being valid, then the scroll bar for the rest doesn't change the buffer.