UpdateMode.AUTO doesn't always refresh screen on resize

[lihop]

UpdateMode.AUTO usually only draws changed cells, but is supposed to redraw the entire screen when the Terminal's parent viewport is cleared (i.e. on resize). But it doesn't always work.

Setting update_mode to UpdateMode.ALL (which redraws the entire screen on every update) does not have this problem, but is bad for performance (more than doubling or tripling the frame time).