Is this a "Magic Trackpad" issue? That is, do you see it fine with a normal trackpad or mouse?

It's not, because this only stopped to work recently and scrollbar is showing fine on any other Phabricator page. For example this task detail page: http://screencast.com/t/b0bTUm41s

I've not been able to reproduce what you describe. If you have more explicit list of things we can do to reproduce, it could help. scrolling has changed recently do anything is possible.

I'm not saying that scrolling isn't working. What I'm saying, and original video attached to task shows, that scrollbar isn't showing up during scrolling. And I guess you must set scrollbars to be hidden by default in OSX interface settings (I'm on Mavericks, no sure if it's issue on Yosemite).

Tested that scrollbar also doesn't show, when scrolling with mouse wheel. So definitely not Magic Trackpad-specific issue.

I dont see any claim by myself above stating scrolling isn't working. I've read the bug and watched the video. I've tried to reproduce the issue with no success. If you have other ideas on how we can reproduce the bug locally, it would be appreciated. I have Mavericks and Yosemite for Mac testing.

I think I found a repro!

@epriestley, it looks like ajax'd content (dashboards) come in after page_height is figured out in JX.Scrollbar. Oddly this is only a trackpad only issue, I can't reproduce it when using a mouse.

Does resizing the window before scrolling fix it? I can probably work around this pretty easily if so.

No, but oddly any DOM manipulation (turning any single rule on/off) made the scrollbar appear.

Hrrrm, peculiar.

This now started to happen on diffusion's commit detail page, when I start to scroll before diffs from all files changed in the commit is loaded via AJAX.

I guess if before you start scrolling enough AJAX content has loaded to change content height on the page to make it larger than window height, then JX.Scrollbar might be working properly.