+++ This bug was initially created as a clone of Bug #1832379 +++

STR

1. Set mail.operate_on_msgs_in_collapsed_threads = false (important!)
2. Select top message (or any contained message) of an expanded thread
3. Press Arrow left to collapse the thread
4. Or click twisty to collapse the thread

Actual

• nothing: It's impossible to collapse the thread whilst one of its messages is selected.
• This is a significant UX impediment for consumers of this pref: In order to collapse any single thread, they must first select a message outside the thread and then click the twisty.
• Keyboard users are unable to collapse a single thread (accessibility issue).

Expected

• Allow collapsing a thread via Arrow left on top message or clicking the twisty.
• 102 has correct behaviour.

This is related to bug 1832379 and can probably be fixed there, but let's track the different symptoms here until that's done.

When I remove attachment 9319122 [details], it works again.

I don't understand the need for ux-implementation-level or ux-mode-error - it's just a regression that can be fixed without further justification, not an intentional new or changed UI design.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/32c2996bde4e
Only update thread pane selection when it's needed. r=mkmelin

Comment on attachment 9336473 [details]
Bug 1832383 - Only update thread pane selection when it's needed. r=#thunderbird-reviewers

[Approval Request Comment]
Regression caused by (bug #): bug 1829491
User impact if declined: can't collapse threads in some circumstances
Testing completed (on c-c, etc.): landed a week ago
Risk to taking this patch (and alternatives if risky): removes some code from a critical function, but it's code that wasn't originally there anyway

Comment on attachment 9336473 [details]
Bug 1832383 - Only update thread pane selection when it's needed. r=#thunderbird-reviewers

[Triage Comment]
Approved for beta

Thunderbird 115.0b2:
https://hg.mozilla.org/releases/comm-beta/rev/971ab3bd1623