ViewportTexture loses track of its Viewport all the time. What am I doing wrong?

Question

Whenever I try to use a ViewportTexture (to get a 2D UI on a texture used in 3D), the texture kind of just works once, clears and then stops updating. I've checked out the "guiin3D" demo project which seems to work, but moving things around a bit and saving that eventually broke as well and I couldn't get it back to working.

An example of what I'm trying to do:

1. Create an empty scene with a Spatial as root
2. Create a Viewport as child, set size to 100, 100
3. Create a Label with a the text "TEST" as child to the Viewport
4. Do "Save Branch as Scene" on the Label to make a scene of it, let's call it "UI"
5. Create a MeshInstance with a QuadMesh and a new SpatialMaterial
6. Set the Albedo texture of the new material to a new ViewportTexture, and pick our Viewport
   You should now see "TEST" (upside down) on the quad.
7. Switch to the UI scene and just save it (ctrl-S)
8. Switch back to the original scene, the quad will now be black

Inspecting the viewport texture shows it's 0x0, and in the output window it throws an error:

scene\main\viewport.cpp:106 - Condition ' !vp ' is true. returned: Size2()

Basically it seems like the ViewportTexture totally loses track of the Viewport, and at this point there's no going back except re-creating the ViewportTexture and the new one gets lost just as easily.

When in the "broken" state, if I clear the Viewport Path of the texture, click Assign and pick my Viewport again, I get this path:

"/root/EditorNode/@@5/@@6/@@14/@@16/@@20/@@24/@@25/@@26/@@42/@@43/@@50/@@51/@@6456/@@6322/@@6323/@@6324/@@6325/@@6326/@@6327/Spatial/Viewport"

Which... doesn't look very reasonable (and also obviously only works within the editor and, again, only until the UI scene is edited.)

Comments

I think every node is already a viewport, so adding a viewport node creates two.

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Not sure exactly what you mean, but I do want two viewports in the sense that one renders an UI to a texture, and the other (implicit) one renders the 3D scene to screen.

Answer 1

Go to the viewport path settings and check on "Local To Scene". That should fix the error.

I was trying to do something similar with rendering a UI progress bar to a Sprite3D texture, and checking that managed to fix it.

For some reason, though, the compiler will report a different set of errors, but still works completely normally and the viewport works correctly.