When calling QLabel::setPixmap() with a 1bpp pixmap (i.e. a bitmap), and that pixmap didn't have a mask set, QLabel would then set the pixmap as its own mask. This seems to be a no-op due to how QPixmap::setMask is coded: void QPixmap::setMask(const QBitmap &mask) { // ... if (static_cast<const QPixmap &>(mask).data == data) // trying to selfmask return; } Moreover, in order to convert the pixmap to a QBitmap, the code would just straight downcast it, triggering UB (if the input to setPixmap wasn't a QBitmap to begin with). I *guess* this was done this way to avoid a QBitmap::fromPixmap call, which however is not expensive at all if the pixmap is already 1bpp, which QLabel::setPixmap checks explicitly (before attempting to mask the pixmap). I don't know the historical reasons for the code to have the shape it had (and the code history is from before open governance). So get two birds with one stone: remove the no-op and also the UB. Change-Id: Ibab20492c2945bd1d01f98a18b168fabc56292b0 Pick-to: 6.3 6.2 5.15 Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>