Excelsior

Random black areas, but if I hit resync, it instantly goes away.

Mine does the same.

I don’t know how to fix it.

You set your display width larger than what the server sends. If you move faster than the server can keep up, there's no buffer of already-sent tiles so you miss them. Hence black.

1000x700 is a reasonably good compromise between large view window and rarely blacked tiles.