The CSS in modules/base/styles/actionbox.less that styles a flyout breaks if its arrow points to the west: in both Chromium and Firefox, the browser window gets ~9000 pixels taller and if you scroll down there's a visible dot at the bottom.
The dot comes from the tipsy-arrow:after selector in modules/base/styles/mixins/arrow.less that has
content: '.'; text-indent: -9999px;
The big negative text-indent pushes the '.' offscreen without creating visible content beyond the normal scroll area; but in conjunction with gravity 'w' which rotates the arrow -90deg, the dot appears in the visible browser content a long way down.
The anonymous editor warning (gerrit 117846) is the first Flow flyout positioned to the side (the default is north), so we haven't noticed this before. You can reproduce with existing flyouts by changing their gravity: go to a short topic e.g. https://www.mediawiki.org/wiki/Talk:Sandbox?workflow=rq3egbkuhi9027u8, click [...] to bring up an action menu, and in the browser's inspector window find the <div class="tipsy-tipsy-n"> at the end of the body tag and change the latter class to "tipsy-w".
There are other glitches with the tipsy flyout arrow positioning:
- The undocumented nw,sw,ne, and se gravities don't position the arrow properly.
- The border bevel doesn't appear on the side of the arrow exactly as in mockups.
- Sometimes a line is visible cutting the arrow off from the main part of the flyout.
Much of this we can leave for fixing when the flyout design migrates to core Agora, tracked in https://trello.com/c/vr0Oai8G