OSX 10.10.2 GTX980
Short description of error
Tweaking a node in compositor clears the backdrop before showing the new tiles so you cannot compare before/after comp easily.
Exact steps for others to reproduce the error
Open the attached file, render and tweak the RGB node. In 2.74 you see that the new tiles simply replace the previous result, in current master it empties the backdrop before showing the new tiles.