PostStream: Fix minor load more issue

[w-4]

When we scroll to a post that is already loaded, the load more button might show and not trigger.
To replicate (in beta.14) go to the start of a long enough discussion and run app.current.get('stream').goToIndex(19).

Screenshot
before the fix:

Confirmed

• Frontend changes: tested on a local Flarum installation.

[askvortsov1]

Oh my goodness, this issue has been driving me crazy during testing! Although I'm unsure about the placement of the onscroll call relative to everything else: that essentially wipes out the results of using updateScrubberHeight vs this.updateScrubber. Perhaps it might be better to factor out the "load more" checks into a separate function like we've done with calculatePosition and updateScrubber so it can be called independently?

Also, you accidentially commited dist changes 😛

[askvortsov1]

When testing locally, app.current.get('stream').goToIndex(19) on a long discussion with these changes results in repeated scrolling to and from the scrolled post.

[w-4]

This is might be due to #2384. Let's merge that in first and check again.

[w-4]

We could later also check if running app.current.get('stream').goToNumber(20) works. But I'd also wait for #2378 to be merged. It removes the m.route.set call in DiscussionPage postionChanged that could lead to an additional goToNumber call.

[askvortsov1]

Tested it again, and it works correctly with app.current.get('stream').goToIndex(19) (yay!).

app.current.get('stream').goToIndex(20) doesn't work (it does nothing), but that behavior is the same on master, so that's a separate issue (probably an off-by-one somewhere)

The only nitpick I have is that I think loadPostsIfNeeded would be a better method name than loadPostsWhenNeeded

[w-4]

Tested it as well, and it works. Also fixed the app.current.get('stream').goToIndex(20).

[askvortsov1]

The goToIndex(20) issue was already fixed in 09e2736, so I don't think we should duplicate it here (especially since the PR is about another topic), but otherwise LGTM!

[askvortsov1]

Tested locally, works as intended