The OOUI hack to prevent scrolling while dialog is open causes pages to scroll when dialog opens (added in 4d87bde4495e5204b722d2154885f9c127ad30e3 / https://gerrit.wikimedia.org/r/#/c/179598/).
To reproduce:
- Edit a long page using VisualEditor, such as https://en.wikipedia.org/wiki/Skyfall
- Scroll far down and open a dialog (for example, for one of the navbox templates; reference list is not a good option because the column highlights in Chrome will mess things up, T52036)
- Observe that window scrollbar disappears.
- Observe that the page noticeably scrolls up as it does; in extreme cases, the template itself might go offscreen.