I suppose you want to make the following modification (I checked the GIT : still not in !)
juce\src\native\windows\juce_win32_Windowing.cpp line 1166 (and following)
if (rects->right <= x + w && rects->bottom <= y + h)
// (need to move this one pixel to the left because of a win32 bug)
// Microsoft seems to have fixed it since !
const int cx = jmax (x, (int) rects->left /*-1*/);
contextClip.addWithoutMerging (Rectangle<int> (cx - x, rects->top - y, rects->right - cx, rects->bottom - rects->top).getIntersection (clipBounds));
Took me a while to trace it, but caused huge performance issues on my app !