PieMenu: Ensure popup menu stays within viewport

[dl3sdo]

The popup pie menu would be clipped when overflowing the screen edges. To avoid clipping, use the existing dead code to shift position.

[dl3sdo]

I'll either add an Q_UNUSED(pos); or modify a copy of pos.
Actually it does not make much difference:
While pos is captured when the user pressed the mouse button, QCursor::pos(); is captured after the event has been received. I assume no-one to right click while doing a rapid mouse move.

[dg0yt]

Hm, how is popup(pos) intented to work?
I assume it should show the popup next to pos; i.e. use that pos everywhere; do not use QCursor::pos() at all.

[dl3sdo]

MapWidget::_mousePressEvent() calls context_menu->popup(event->globalPos()); (with context_menu = new PieMenu(this);).
See QMouseEvent::globalPos() const
Ok, it seems that you prefer the modification of a copy of pos.

[dg0yt]

To be revised after #2513.
Fun fact: I can skip the calculation and still the popup will be positioned on the screen...