QPushbutton with an icon behaves incorrectly in a mixed-dpi environment.  In particular,

QFusionStyle::drawControl draws the icon for CE_PushButtonLabel like:

QPixmap pixmap = button->icon.pixmap(button->iconSize, mode, state);

but QCommonStyle::drawControl does it like:

QPixmap pixmap = button->icon.pixmap(qt_getWindow(widget), button->iconSize, mode, state);

Because Fusion style is missing the QWindow* parameter to QIcon.pixmap() call, the global qApp->devicePixelRatio is always being applied, even on widgets / screens where this is not appropriate.   In my case, the QIcon draws correctly on a 4k screen but not on a secondary 1080p screen.

I have attached 2 screenshots that demonstrate the issue.  I created a QIcon with pixmaps of both 90x90 and 30x30.  With style Fusion, QIcon:pixmap selects the 90x90 icon (which is not correct, and it's rendered jagged).  Without fusion style, the correct 30x30 pixmap is chosen.