Fix DiscussionListPane jumping around (when going from discussion to discussion)

[w-4]

Fixes the following issue:

1. Go to https://nightly.flarum.site and open a discussions.
2. On the discussion page scroll the pane a little.
3. Click on a different discussion and the pane jumps back to the top, not keeping the scrolled position.

Confirmed

• Frontend changes: tested on a local Flarum installation.

[askvortsov1]

Thank you!

[clarkwinkelmann]

Is that paneScrollTop global variable exclusive to this particular feature? If so I think its name should reference the component that makes use of it, so we're certain it does not get used by something unrelated in the future or by extensions.

[w-4]

I renamed the variable. Also, after implementing https://github.com/flarum/core/issues/2413 caching of the scroll position won't be necessary anymore.

[clarkwinkelmann]

I haven't followed well enough. Will this still be necessary? Should I review again?

[askvortsov1]

@w-4 Just to clarify, you're saying that with flarum/issue-archive#176 the change in this PR won't be needed, right? (since we won't be rerendering the panel completely anyways). If so, maybe we shouldn't be merging this?

[w-4]

@askvortsov1 Yes, only the if condition if (!app.previous.isDiscussion()) would be needed. I'd merge it in, it fixes the bug, and flarum/issue-archive#176 can be done after stable.