When using QWidget::repaint() on linuxfb, the widget isn't immediately repainted on the screen.

This seems to happen because flushing is asynchronous: QFbWindow::repaint() calls QFbScreen::setDirty, which then schedules an event-driven update:

https://codebrowser.dev/qt6/qtbase/src/platformsupport/fbconvenience/qfbscreen.cpp.html#_ZN9QFbScreen8setDirtyERK5QRect

This should be synchronous instead, as the user requested it. (Do note that the call to repaint() comes from Qt's own code! For instance, QPushButton repaint()s on a mouse release, so that when clicked() is emitted one can temporarily block the event loop with a blocking call, but the button is supposed to be repainted lifted.)

This is a regression from Qt 4 / QWS.