WebGLBackground: `renderer.autoClear` has no effect when an XR session is active

[mrxz]

Description

Since r152 the renderer.autoClear flag is effectively ignored when in an XR session. This seems to be a result of changes made as part of #25781. The code in WebGLBackground now force clears if there is an XR session.

Relevant code: https://github.com/mrdoob/three.js/blob/dev/src/renderers/webgl/WebGLBackground.js#L47-L66
(note that the switch case is exhaustive as per WebXR spec, and all cases set forceClear)

This causes a problem when the frame is rendered using multiple renderer.render() calls. Setting renderer.autoClear = false no longer suffices in that case. It can be worked around by setting the individual autoClearColor, autoClearDepth and autoClearStencil, as these are still honoured, but I believe the behaviour differing between non-XR and XR is a bug (or at least not intuitive).

Reproduction steps

1. Disable autoClear
2. Render a frame with more than one render() call (e.g. clear(), render(object1), render(object2))
3. Start an XR session and see only the result of the last render() call

Code

renderer.autoClear = false;

renderer.clear();
renderer.render(object1, camera);
renderer.render(object2, camera);

Live example

Example: https://jsfiddle.net/br1nygdk/2/

The example shows two spheres; one cyan and one magenta. When clicking 'Enter VR' only the magenta sphere is visible, whereas both should be visible. Setting autoClearColor to false works around the issue.

(Note: it's not possible to enter VR in the fiddle on a Quest, but the immersive-web-emulator can be used as well)

Screenshots

No response

Version

r152

Device

Headset

Browser

Chrome

OS

Android

[marwie]

Just came across this here as well and came to the same workaround/conclusion, setting autoClearColor to false now as well