Menu position of own toolbar is not correct

I've created an own toolbar that can be docked on every edge on the monitor.

When the toolbar is docked on an edge, the required place is reserved by calling some WinAPI functions so that other windows and icons on the desktop are adjusted to the new available screen geometry.

The toolbar is made up of QToolButtons which can show a QMenu.

If the toolbar is docked on the left or right edge, the position of the menu popping up is in my opinion incorrect (see attachment left.png). This is because I reserved the space for the toolbar through the WinAPI calls and the QMenu adjusts on that. Between the toolbar and the QMenu is one pixel space, so I cannot browse on the menu since I go on the one pixel space and the QMenu disappears.

But if I dock the toolbar on the topor bottom edge, everything is alright (see attachment top.png). I also have reserved the space for the toolbar (here on the top edge), but that doesn't effect the position of the QMenu