Panning in camera view makes the border to be modified, which was causing
the Cycles display to believe the rendered result is unusable.
The solution is to draw the render result at the display parameters it was
updated for. This allows to avoid flickering during panning, zooming, and
camera FOV changes. The suboptimal aspect of this is that it has some jelly
effect, although it is on the same level as jelly effect of object outline
so it is not terrible.
Differential Revision: https://developer.blender.org/D12970
31f6e78370