Emulator

Do you have a two monitors of different sizes? Qt seems to have problems with DPi changes.

They're the same model. Both 1920x1080

I was able to repro this problem. I have the exact same setup - 2 1920x1080s. As described, opening the settings dialog on one screen, closing it with the "X", moving Dolphin window to other screen, then opening settings dialog renders all white. Now, if you take this white dialog and move it to the other screen, the controls appear! If you resize the white dialog, the controls appear.

I did some investigating in the code, and my conclusion is this is a Qt bug. I compared the Settings dialog to the Controls one, since this problem doesn't happen with Controls. I stripped down Controls so that it has nothing but the Close button on it, and then it happened with Controls as well. If I add an empty tab page widget, then the problem goes away -- this is true for the Settings dialog as well. If you add a dummy empty tag page widget to it, the problem disappears. So basically, it looks like Qt is refusing to paint the dialog when it opens on a separate screen when certain widgets are not present. Keep in mind that Dolphin creates these dialogs and hides them on start, then simply shows/hides them when opening/closing them, rather than re-creating them. I think this isn't a typical use-case; however, Qt should handle it.

That's as far as I've gone. I have very little Qt experience, so I'm not sure how to proceed. Perhaps a more experienced Qt programmer can actually debug this deeper in Qt to understand it?

I'm sorry, I mixed up Controls and Settings dialog in my previous comment. Swap those two in your mind as you read the above!

Well I just switched out my mainboard literally last friday. So I can't say anything about the bug on my old setup with the current dev version.

My old mainboard is GA-B85M-D2V Don't know what revision with an dual core i3-4360 CPU @ 3.7GHz, 8 GB of DDR3 RAM and a GTX 1060.
My current specs are an Asus Prime Z370-A mainboard with an i3 8350K 4x @ 4.00GHz, 16 GB of DDR4 RAM and a GTX 1060.
I have 2 BenQ GL2450 1920x1080 Monitors, one connected via HDMI and the other via DVI and use Windows 10 Education.

Well I just tested 5.0-11653 and 5.0-11655 (with 11655 being the QT update).

The bug did happen on 5.0-11653 and didn't happen on 5.0-11655.