[3.x] ViewportContainer unexpectedly turns off physics interpolation

[lawnjelly]

Tested versions

3.6 from beta 3 (2D physics interpolation)
Doesn't occur in 3.5 and prior to 2D physics interpolation

System information

All

Issue description

In a physics interpolated game, when using ViewportContainer, objects below the ViewportContainer are unexpectedly not physics interpolated.

This issue was isolated from #91918 (and found by @elvisish )

Steps to reproduce

• Create ViewportContainer
• Create physics interpolated gameplay below the above node
• Switch on physics interpolation in project settings
• See no physics interpolation

Minimal reproduction project (MRP)

Press return to toggle on / off the physics interpolation mode of the ViewportContainer.
viewport_interpolation_bug.zip

Discussion

Unsure whether to class this as a bug or documentation as yet, but probably something I can fix engine side to make more sensible.

The problem stems from 2D physics interpolation, because Control now defaults to PHYSICS_INTERPOLATION_MODE_OFF. This is because user interface elements typically are not desired to interpolate (they can still be interpolated but they must be explicitly set to e.g. PHYSICS_INTERPOLATION_MODE_ON or PHYSICS_INTERPOLATION_MODE_INHERIT).

In the particular case of ViewportContainer (and possibly other controls containing a viewport with an interpolated scene), this has the side effect of propagating the physics_interpolation flag to be OFF for the scene tree branch below the viewport.

There may be a simple fix to this (rather than documentation), such as defaulting Viewport itself to PHYSICS_INTERPOLATION_MODE_ON rather than PHYSICS_INTERPOLATION_MODE_INHERIT, thereby resetting physics interpolation to the default state below any viewport.

[lawnjelly]

Fixed by #92152.