debugging transparency

Debugging someone’s spaghetti is a drag, but here I am once again!

Looks like a bug w/ white blending. White is meant to [partially w/ blend or alpha] rub out everything under the marker existing on the screen. It works for single HDC, but changing device context, and hence probably a lot of other [gl and windows] stuff causes white rubout to fail for previously drawn items and only work for items newly drawn in most recent context. I suspect that reusing a single device context for zooming and sizing and havent yet dismissed it, but was hoping for some input from this forum.