Sergey Sharybin
00d476976e
The issue was caused by combination of following factors: - Blender Internal viewport render can not distinguish between which parts of main database changed, so it does full database re-sync when anything is tagged for an update. This way, if any NodeTree (including compositor) is changed, Blender Internal viewport is tagged for full render database update. - With old dependency graph, scene-level drivers are evaluated on every iteration of scene_update_tagged, even if nothing is tagged for an update. This causes compositor drivers be evaluated quite often. - Driver evaluation checks whether value was changed, and if so it tags corresponding ID type as updated (this is what was telling viewport to do render database update). This check was quite stupid: current property value was checked against the one coming from driver expression. This means, if driver value is outside of the hard limit range of the property, the property will always be considered updated. The fix is to compare current property value against clamped value from the driver.