This userscript lets you ctrl+click a message to hide/show its replies, i.e. collapse that message's (sub-)thread. (n.b. messages' replies can be preemptively hidden before they have any, i.e. don't randomly ctrl+click messages)
Also, note that this script is currently in quite an alpha state (see Bugs section below)
To run it you'll probably need to install a userscript manager. Good ones include Greasemonkey, Tampermonkey, and Violentmonkey. Despite being a stalwart Greasemonkey user, I'd suggest Violentmonkey. (also, why are they all monkeys ?)
Once you've installed one, click this link to install the script. You should be asked if you're sure you want to install the script. Thereafter, updates should be automatic (at least, theoretically; I have yet to actually confirm this). You should also be able to manually update it in Tampermonkey & Violentmonkey.
The input bar will disappear, if you jump to an unread message that is inside a collapsed thread, and in some other cases.
Messages will appear to become unread again after un-collapsing a thread.
The "thread collapsed" indicator shows above instead of below the input bar.
Messages without replies being prematurely collapsible is a "feature" and not a "bug".
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