The issue is that timeline snapping code does not set the transform
`values` to a rounded frame delta, instead, it adds the rounded distance
between current source and target points to the unrounded `values`
float, leaving it unrounded.
Since snapping code is only run on some `transform_modal` calls due to
"time base quirky code" in `transform_snap_mixed_apply`, sometimes
frames briefly show mouse input added to `values` causing the strip to
get transformed away from the snap target before the snap code is run
once again.
To fix this, set transform `values` to the exact rounded snap target
frame diff (like in node snapping) instead of expecting it to get
properly rounded for every call. Do this by changing
`tsnap->snap_source[0]` to the original snap source on `transformInit`,
not the updated snap source.
Pull Request: https://projects.blender.org/blender/blender/pulls/123217
7b6ae7abbb