Prefer QRegion::operator+=
`QRegion::operator|=` has some optimizations but it basically boils
down to
QRegion result(*this);
result.detach(); // it will make a copy because this is shared
result.d->append(rect);
return result;
On the other hand, `QRegion::operator+=` tries to add the new rect
in-place.
Edited by Vlad Zahorodnii