[REG: 5.5.1->5.6.1] QDockWidget cannot be properly docked on a secondary screen that has negative position

When undocking a dock widget and trying to drag it to another docking position, the docking fails and the dock widget is restored to the initial position. This happens also if the widget is floating and then moved to another position - it will restore to the original docking position.

Reproducing the bug requires 2 monitors where the secondary monitor is on the left side (or above) of the primary monitor - thus has "negative" coordinate space.

Steps to reproduce:

• Run attached sample
• Undock one of the dockwidgets and drag it to left dock area of the main window
• Try to dock the dragged dockwidget on the left side docking area
• EXPECTED: dockwidget is properly docked on left side
• ACTUAL: dockwidget is restored to the right side docking area

Scenario #2 steps:

• Run attached sample
• Undock one of the dockwidgets and leave it floating
• Try to dock the floating dockwidget to left docking area
• EXPECTED: dockwidget is properly docked on left side
• ACTUAL: dockwidget restores last remembered docking position on right side, it does not restore to floating state