[Bug] Moving tabs with tree structure to start flattens the tree structure of other tabs

[codrinbucur]

Abstract

Steps to reproduce

1. Start Firefox with clean profile.
2. Install TST.
3. Create multiple tabs with multi-level tree structures
   4a. Move one tab with multi-level to start
   4b. Closing and reopening Firefox can also lead to flattening of tabs with multi-level tree structures. Especially if Firefox hangs and it is killed but also after normal restarts or browser updates

A <- Tree Structure Destroyed (2)
\- B (collapsed)
C <- Tree Structure Destroyed (2)
\- D
E (selected) -> Move to Start (1)

Expected result

Multi-level tree structures of all the other tabs should be left intact during any operations to other tabs or browser restarts

Actual result

Multi-level tree structures of some of the tabs get fully flattened without any option to restore them

Environment

• Platform (OS): MacOS Tahoe 26.4.1 (25E253)
• Version of Firefox: 149.0.2 (aarch64)
• Version (or revision) of Tree Style Tab: 4.3.1

Currently on 4.3.1 but been witnessing this tab structure loss since 1.5 years with various versions of Tree Style Tab and Firefox. It is is very disruptive issue which I was hopping would be fixed by now

It would be nice if the tree structure can be "saved" and "restored" if things happen to it

[thnee]

I have also had problem with tree being flattened recently.
I have around 1000 tabs, with some very deep trees.
Suddenly some sub-trees from inside a root have been moved out to the top level.
The sub-tree itself remains intact, the children are in the same order.
After this happens, the tabs remain stable in the new position for some time, even when closing and reopening the browser.

It seems like TST still knows about the correct position somehow though, because:
When closing an reopening these tabs, they kind of move back into their original position.
But this in turn causes visual bugs, where the tab is shown in the wrong location in the list, and opening a tree reveals the tab somewhere else in the list.

This problem happens quite rarely, but now it has happened multiple times over several days/weeks, and eventually my deep trees have become a flat mess.
I have no idea what is causing this, but it seems to happen when moving sub-trees by dragging them around.
It seems like this is a new bug in recent weeks. I've been using TST for years without noticing this problem before.

Example (simplified)

# Before
A/
  A1/
    A11
    A12
  A2/
    A21
    A22
B/

# After
A/
  A2/
    A21
    A22
A1/
  A11
  A12
B/