Here's a good place we can use for exploring how to improve the performance of our new TreeViewListbox.

These are some fairly low hanging fruit we can start adding:

• If we're dealing with the subjectCol, use cellTextForColumn only once to set the text in the column and the aria-label for the row. Fetching the subject text twice is expensive.
• We should have a single <template> with all the cells of the message thread, and when constructing the row on connectedCallback(), extract those cells from the template and add them to a DocumentFragment based on the Columns::ordinal, and then append it all at once. Avoid replacing or creating new DOM.Nodes inside the set index() method.

Anything else that comes to mind?

try run: https://treeherder.mozilla.org/jobs?repo=try-comm-central&revision=656b411ec2d507f370ac8e51fe22998a7aa23a89

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/26f62abc23bf
Clean up the tree view listbox code for the about3Pane. r=darktrojan

Comment on attachment 9317172 [details]
Bug 1814320 - Clean up the tree view listbox code for the about3Pane. r=darktrojan

[Approval Request Comment]
Regression caused by (bug #): Supernova

Comment on attachment 9317172 [details]
Bug 1814320 - Clean up the tree view listbox code for the about3Pane. r=darktrojan

[Triage Comment]
Approved for beta

Thunderbird 111.0b1:
https://hg.mozilla.org/releases/comm-beta/rev/1b17ec1019be

Converting this into a meta bug to avoid crossing versions.
Blockers of this have been added and Geoff is tackling the most obvious downfalls.