[REG 5.6.1->5.10.0] QDockWidget issue with moving by mouse

Bug manifests itself by inability to move QDockWidgets with custom titlebar properly by mouse. This occurs if user:

a) clicks on the edge of the title bar (top, left or right edge) and drags cursor outside of widget or

b) clicks anywhere on the title bar and moves mouse fast upwards (or sideways)

This bug can be reproduced with this example: qt5\examples\widgets\mainwindows\mainwindow\mainwindow.pro with "Blue Dock Widget".

Steps to reproduce:

• Run the example
• Undock Blue Dock Widget
• a) Click on upper edge of the title bar
• Move mouse upwards (speed doesn't matter)
• ACTUAL: Blue Dock Widget stays on old position
• EXPECTED: Blue Dock Widget follows mouse cursor

• b) Click and hold left mouse button over the title bar without moving it
• Quickly move mouse upwards
• ACTUAL: Blue Dock Widget stays on old position
• EXPECTED: Blue Dock Widget follows mouse cursor

 Attached video contains this behaviour.