Doing that is quite expensive (70ms+ on my machine), so check a couple of pixels first before actually doing that.
I never actually had a case where alpha channel would mismatch, small previews were always fine, so please test
The code was added to fix a bug, https://bugs.kde.org/show_bug.cgi?id=362450
That seemed to be caused (or triggered?) by small rectangular selections, so perhaps this sanitization condition could be changed to detect whether the preview area is smaller?
@davidre said something about that we don't show small thumbnails anymore anyway or something