Here is a way to get the jumping separator.
Install Tinderbox from a .dmg. Start the fresh installation. Move the separator. Note that it behaves correctly. Grab a window-corner and enlarge window. Grab separator and move it, let go and it jumps back.
The above is repeatable and I have it happen on both my iMac(26.0.1) and Macbook(26.1 Pub beta).
As soon as Tinderbox jumps back to expected functionality regarding the separator (I have not found a repeatable pattern for what causes this) I have not been able to get it to fail again.