Window decorations do not scale correctly when moved across monitors

[gkcgareth]

I have a multi monitor setup on my windows 10 host. One of the screens has a much higher DPI than the others so I ran it at native resolution and use scaling to keep the same visual appearance as the other. This is the microsoft recommended approach, and is the default configuration for this device.

Under this setup, any virtual box window always has the scaling for that first monitor applied - when I move any VB window to the other display, the title bar is twice as big as it should be, and windows that open there are sized twice as big as you'd expect. Window contents appear to scale fine.

This doesn't happen with any other application that I've found so far. All other apps scale correctly depending which monitor they on.

An easy test is to have a common edge between the monitors and move a window so it's half on each monitor, then move and back and forth so the majority falls on different monitors - you will see the scaling take affect as you do, and notice that VB windows behave differently to other applications.

To reproduce:

1. Set up two monitors, and configure the main monitor with a scale factor of 200%
2. Open up the main VirtualBox window, and move it between the monitors - observe the size of the title bar as you do so. Slowly move until 50% of the window is on the other monitor, and observe changes. Repeat these steps with a non VB window (e.g. notepad, explorer) to see the difference