Issues with multi-display setup (SoftMaskTex empty)

[Propel-antonin]

Describe the bug
There seems to be an issue with multi display setups. (When you're got multiple cameras, with multiple UI canvas rendering to different display indices).

Depending on which canvas is the "main" display, and which one aren't, certain UI elements will get entirely black UI mask textures and disappear. The bug seems to be really inconsistent and hard to reproduce.

To Reproduce
Steps to reproduce the behavior:

1. Duplicate "ScrollRectTest" scene.
2. Duplicate every camera/canvas, assign one of each to display 2
3. Embed another mask (e.g. another scroll rect) within the display 2 scroll rect (see fig 1)
4. Press play
5. Duplicate the game view, one on display 1 and one on display 2
6. Click on display 1
7. Notice how one of the mask on display 2 might disappear (see fig 2)

Fig 1:

Fig 2:

Expected behavior
The mask on the second screen should not disappear once the first screen is focused. This bug also happens in a build.

Environment:

• Version 3.5.0
• Platform: Both Windows Editor & Build
• Unity version: 6000.0.58f2

Additional context
Add any other context about the problem here.

[Propel-antonin]

I have forked the project and will attempt to fix the bug on my own, but please take a look yourself if possible.

[Propel-antonin]

It seems like Screen.width / Screen.height is used often.
I will try to see if changing everything to use the Display API will fix everything.

var display = Display.displays[displayIndex];
return new Vector2Int(display.renderingWidth, display.renderingHeight);

EDIT 1: Currently running into this issue with the setup above. Display.displays.length is always 1 in editor.
EDIT 2: Figured out a way around that by caching the resolutions of non active windows.

[Propel-antonin]

Nested graphics in the editor seem to now work, I'll test to see if everything works fine in the build

[Propel-antonin]

Everything seems to be fixed in the build.

You can take a look at how I fixed the issue here:
Propel-antonin@8737295

Made a pull request here: #265

[Propel-antonin]

There's a bug with my pull request, I will patch later.
if(self.canvas.worldCamera != null && self.canvas.worldCamera.targetTexture == null) in GraphicExtensions.cs 118