[Enhancement] hide collapsed tabs and introduce a key shortcut to collapse/expand

[polysquirrel]

Abstract

Steps to see the situation

This is an alternative (or complementing) proposal to #3324

A (focused)
 \- B (collapsed)
 |- C (collapsed)
D 
 \- E
 |- F

Expected result

Let's define the toggle as ctrl+space for 'collapse/expand all' and ctrl+shift+space for 'collapse/expand recursively'.

Pressing either ctrl+space or ctrl+shift+space should create the following tree:

A (focused)
D
 \- E
 |- F

Additionally, tabs E and F are hidden from the top tab bar.
In this configuration, ctrl+pgdown brings focus to D, and following ctrl+pgup brings focus back to A.

[piroor]

There are some my past comments describing risks when TST hides tabs in Firefox's native tab bar:

#1831 (comment)
#1371 (comment)

In short: Tabs hidden when they are just collapsed will introduce compatibility issues with other tab addons and Firefox's native behavior (because hidden tabs are treated as unexisting by Firefox itself.) Thus I decided that I don't introduce this change.

[github-actions]

This issue has been closed due to no response within 7 days after labeled as "wontfix", 7 days after last reopened, and 7 days after last commented.