Make PostStreamScrubber work for Posts that have top margin

[w-4]

Basically the same as #1944. I now know what that PR was fixing. When Posts have a top margin the scrubber will bounce around.

To replicate go to https://nightly.flarum.site, set margin-top: 50px for .Post in the dev console. See scrubber bouncing back and forth when scrolling.

Not a priority, but should still be fixed I think.

Edit: New commit fixes #1897

Confirmed

• Frontend changes: tested on a local Flarum installation.

[askvortsov1]

Hmm, I remember I was looking at this back when doing #2160, but didn't include it for some reason, don't remember why though. I'll test it locally

[askvortsov1]

Tested locally, this fixes the issue and doesn't seem to introduce any others. Thanks!

[clarkwinkelmann]

That looks good, but would it maybe be prettier to use two number variables and no boolean?

Pseudo-code:

indexFromViewPort = null;
$.each(() => {
    // [...]
    if (!indexFromViewPort) {
        indexFromViewPort = parseFloat(...) + visibleTop/height
    }
    // [...]
})
// [...]
index = indexFromViewPort || $items.first().data() || 0

EDIT: I fixed my pseudo-code, the if statement was missing

[askvortsov1]

That looks good, but would it maybe be prettier to use two number variables and no boolean?

That looks nice! Could also get us a bit closer to the implementation needed for #1897, so I like it

[w-4]

Will do, we need to check against null though both times, since zero can be a valid index.

[askvortsov1]

Re-tested locally, confirmed that it still works.

[dsevillamartin]

Looks like it fixes both issues mentioned (only one is an actual issue in the repo). Tested locally 👍