Not sure when this started, tested with vpx. (found thanks to #692)
The opengl window renders the update only on the top-left portion of the screen which corresponds to the downscaled dimensions. The non-opengl rendering works fine.
Good news: v0.14.x is not affected by this, but trunk r7803 is.
Bisecting by visual inspection (..) using a server running like this:
XPRA_SCALING_HARDCODED=1:3 xpra start :10 --start-child=glxspheres --no-mmap
Hardcoding the scaling to ensure it is enabled, and disabling mmap which bypasses encodings.
So this was caused by #679.
Fixed in r7804: we must paint the screen using the unscaled dimensions!
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/693