[KK/KKS][Timeline] Plugin crashes when Alt+click moving keyframes (under specific condition)

[AX-MMD]

For Timeline 1.5.5.1

Describe the bug
Timeline crash when moving keyframes (with Alt+click and move) and the final destination of a moved keyframe coincides with the previous position of another currently moving keyframe. After the crash some Timeline functions may become unusable until Studio is restarted (usually Left-Click selection and Alt+click Move just won't register anymore).

This happens specifically when moving forward with Alt+click Move, especially when holding Shift to snap to an exact time.

This happens with or without ShalltyUtils and its Timeline "Performance mode", though the logged error is slightly different (logs attached below).

It does not happen:

• When Alt+click moving backwards
• With cut-paste
• With the "Drag at current time" button, because it won't allow to move a keyframe on top of another keyframe

Expected behavior
Alt+click Move displaces the keyframes forward and handles whatever "ghost" keyframes there might be without crashing Timeline.

NOTE:
This also happens if you move a keyframe on top of another, but I'm not sure what the behaviour should be for that case (squash the keyframe, or don't allow the move?)

To Reproduce
Steps to reproduce the behavior:

1. Place 2 keyframes, at 00:01 and 00:02 (can use the Camera interpolable)
2. Select both keyframes, then move with Alt+click so that the left keyframe lands on the right keyframe's previous location exactly (easier by holding Shift while moving to snap)
3. Timeline crash in log, if you try to select of move the keyframes again it becomes funky

Screencap

20260629-1036-26.1653378.mp4

Additional information

• KK and KKS
• Win 11
• Timeline v1.5.5.1, KKAPI 1.46.1

Error log with ShalltyUtils installed and its Timeline "Performance Mode" enabled
kks_studio_log.txt
Error log without ShalltyUtils installed
kks_studio_log.txt

[ManlyMarco]

Is this a new issue introduced in v1.5.5.1?

[AX-MMD]

Is this a new issue introduced in v1.5.5.1?

No it's been there for a while, I can test older versions to double check

[AX-MMD]

Same issue on 1.5.1 and 1.4.2

[ManlyMarco]

What's the first version with this issue?

[AX-MMD]

What's the first version with this issue?

I can go down to 1.2 and replicate the issue

I can still replicate the behaviour of the bug down to the first version, but lower than v1.2 I start getting Missing method exceptions so not sure it's the same cause

[AX-MMD]

v1.1.2 log for reference
kks_studio_log.txt

[Lusiocc]

I will open a PR for this shortly. I'm verifying my changes right now.

[Lusiocc]

Fix downloadable via https://github.com/IllusionMods/HSPlugins/actions/runs/30171030676/artifacts/8622922350