When panning we require new data from the server to draw the new parts of the screen. This seems to be working very inconsistently. The result is that you have blank parts of the screen until you do something else which will trigger the update.
It is a lot easier to reproduce on touch devices, where you don't have a mouse cursor.
Upstream bug: https://github.com/kanaka/noVNC/issues/517
Fixed in upstream by double buffering: https://github.com/novnc/noVNC/pull/699
We would no longer need frame buffer updates after panning.
Works as expected