if an update for n≥2 rects in a window is requested, seemingly the background is erased for the enclosing rectange but updates are clipped to the actual region only.
grey: the lower80% is a terminal widget which does precise update(…) calls where characters are modified.
the window background is painted with
the terminal widget repaints with:
note that the standard GUI widgets at the top are also cleared but not repainted.
orange: here are some QLineEdits widgets on a window. They are updated on a timer which mostly updates only some widgets because mostly only some values change. seemingly unmodified (and not updating) widgets between updated widgets are cleared and not repainted.
the window background is painted with:
the QLineEdits are not subclassed.