AnimationAction: Fix timeScale reversal jump.

[Mugen87]

Fixed #19151.

Description

The PR fixes a long-standing bug in AnimationAction that happens when the time scale of the animation mixer is reversed during the first run of looped animations.

I've tried to fix this multiple times in the past but wasn't sure about the real root cause. With the help of Claude 4.6, this should be resolved. Claude required multiple attempts but eventually found the root cause in an else branch where a member wasn't correctly updated.

In _updateTime() , the "just started" block (line 781) sets the local variable loopCount from -1 to 0, but this._loopCount is only persisted inside the wrap-around branch (line 847). The non-wrapping else branch (line 857) never writes it back. This means this._loopCount stays at -1 indefinitely until a wrap-around occurs. On subsequent frames, the "just started" block re-executes, and for PingPong mode, the check (loopCount & 1) === 1 evaluates to true because -1 & 1 === 1, causing unwanted time inversion and a visual jump.

The test code from #19151 now behaves correctly and the animation examples should also work as before (tested locally). I think it's worth giving this fix a try!

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.15 85.24	359.15 85.24	+0 B +0 B
WebGPU	623.55 173.47	623.55 173.47	+0 B +0 B
WebGPU Nodes	622.13 173.21	622.13 173.21	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	490.87 119.65	490.87 119.65	+0 B +0 B
WebGPU	697.03 188.44	697.03 188.44	+0 B +0 B
WebGPU Nodes	646.24 175.82	646.24 175.82	+0 B +0 B