Calling popup menu in fullscreen mode make flickering on the screen (DisplayServer regression)

[Chaosus]

Godot version:
f1832a6

OS/device including version:
Win10/GTX1060

Issue description:
Clicking(especially frequent) on these buttons which called popup menus
will cause the whole screen flickering with dark flash.

Steps to reproduce:
Simply run the empty project, toggle fullscreen (Shift+F11), and try to use the buttons showed above.

Single Window Mode should be disabled (of course).

Minimal reproduction project:
It's not recordable with ScreenToGIF unfortunately.

[Calinou]

It's not recordable with ScreenToGIF unfortunately.

You could record your screen using your phone 😛

[Chaosus]

You could record your screen using your phone

It's unprofessionally 😛 but ok - https://drive.google.com/file/d/1RyWLsCaUC53o7dFY3Kt784x45E54iiyB/view?usp=sharing (too lazy now to push it on youtube)

[EricEzaM]

Pretty sure you can attach videos directly to github posts now.

Ps to access the file on drive, I need to request access. Need to make it public

[Chaosus]

VID_20210107_220735.mp4

[akien-mga]

Is that a recent regression?

[Chaosus]

At 41e9028 the pressing these buttons in fullscreen leads to editor window become invisible (which is fixed by 8a2bc6b) so I guess its exist at the point where Juan separated display server (10 months ago). But need to test for sure.

Also found an interesting topic about possible fixing it: glfw/glfw#527

[Chaosus]

After some experimenting I found 2 ways to solve this problem:

1. Append 1 pixel to the height of the window which enters fullscreen mode. The downside of this - the client rect will be changed and this may alter the content.
2. Change WS_POPUP to WS_CHILD for borderless windows. The downside of this is child windows behaved differently than popups - for example, their positions and event processing needs to be modified. They don't close automatically if the users changed window focus.