This is interesting:

> Impossible to set scaling on crtc 59 to 1.000000, error id 2

Error 2 is BadValue and I can reproduce similar on the command line:

$ xrandr --output VNC-output-0 --scale 1x1  # or 2x2, 0.5x0.5

This works in Xorg but seems to always fail in Xdcv with error BadValue
(2). And the same with:

$ xrandr --output VNC-output-0 --filter nearest # or bilinear

Works in Xorg but not in Xdcv.

So fractional scaling failing is expected on Xdcv, but I don't think
those functions are historically needed for integer scaling. Only our
fractional scaling patch tries to use them unconditionally for integer
scaling. So that's both a unique failure to the fractional scaling patch
and also not its fault -- failing to change the scaling matrix is a
failure on the Xdcv side.

In summary we should probably only be trying to fix the fractional
scaling OFF case here. Because that's probably the only mode that Xdcv
can handle in theory.

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https://bugs.launchpad.net/bugs/2020782

Title:
  Xdcv: Changing display scale setting and reverting it fails

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