html5 window resizing goes blank

[Antoine Martin]

The window gets repainted with the updated contents (new size), but then somehow ends up being repainted again with empty contents (white).

Debug paint log:

client draw may_paint_now() paint pending= 0 , paint queue length= 1
client draw do_paint( 2388  bytes of   webp  data  775 x 927  at  0 , 0 ) focused= true
client draw request_redraw for Window(1)
client draw swap_buffers
client draw decode time for  webp  sequence  21 :  5 , flush= 0
client draw may_paint_now() paint pending= 0 , paint queue length= 0
client draw animation frame: 1 windows to paint

[Antoine Martin]

Workaround in r22901: we just force a refresh shortly after the end of the resizing.

I have no idea why sometimes things get wedged, and no time to investigate further.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2321