Display bugs on HiDPI Windows hosts with two displays

[VasilyA]

I'm running VirtualBox on a Windows machine which has two screens: one HiDPI with 200% scaling enabled, and the second one using 100% scaling.

When I launch VirtualBox Manager, it launches with 200% scaling (which is correct), but when I drag VirtualBox Manager window to my second display, it doesn't rescale correctly (see attached screenshots).

What's worse, VM window doesn't scale at all when dragged to the second screen.

This bug was introduced in 5.1.0 release.

[VasilyA]

Virtual box when launched on HiDPI screen

[VasilyA]

VirtualBox when dragged to 100%-scaled screen (the fonts did rescale to 100%, but window decorations are huge)

[VasilyA]

VirtualBox VM window on 100%-scaled screen (the VM window was left with 200% scaling, so that corner is all I see on my 1920x1080 display)

[breyed]

Related repro - a common real-world scenario:

1. Install and run VirtualBox on computer with its Main Monitor at 192 DPI (200%).
2. Sign out of Windows.
3. Correct an external monitor (e.g. by putting laptop into its docking station) so that its Main Monitor is now at 96 DPI (100%).
4. Run VirtualBox. Notice that all the text is HUGE. VirtualBox seems to have held onto the old scaling from when it was first run.