A-Frame mouse and touch controls don't work when <a-scene> z-index is negative

Description:

• A-Frame Version: 1.2.0
• Platform / Device: Chrome on Windows, but suspect it's universal
• Reproducible Code Snippet or URL: Glitch here: https://adaptable-quick-dedication.glitch.me/

A-Frame help here discusses the use of z-index as a way to layer 2D UI elements over an A-Frame scene.
https://aframe.io/docs/1.2.0/components/embedded.html#integration-with-the-2d-page

However based on this advice there is a chance that someone might be inspired to set the z-index of their to a negative value (which is exactly what I did).

Having done that, they will then discover the unfortunate side-effect that mouse & touch controls can no longer be detected in the A-Frame scene, as demonstrated in this glitch.
https://adaptable-quick-dedication.glitch.me/
(press the button 3 times to get to negative z-index values).

I don't understand the root cause, but have observed that when "a-scene" z-index is 0 or higher, then setting a breakpoint onto mouse or touch events in the Chrome debugger reveals various events being triggered on CANVAS. And when z-index is < 0, no events are triggered on CANVAS.

I doubt anyone has a genuine reason to need their "a-scene" to have a negative z-index, but at a minimum this issue should be documented at the docs link above.

It would be nice to have a fix, as even with documentation it will be easy to stumble across this, and the consequences might not be obvious (for me, I was trying to troubleshoot why raycasting wasn't working, and after several hours I finally realized that it was down to this).

Start with the linked Glitch reproduction and verify mouse and touch behavior while changing the a-scene z-index from 0 to a negative value. Read the linked embedded-scene documentation and investigate why canvas events stop firing; done means either the controls work with a negative z-index or the limitation is clearly documented there.