Steps to reproduce:
- Go to an English Wikipedia article on the latest Firefox version on MacOS Mojave or High Sierra.
- Make sure there's a donation banner on top.
- Scroll to verify that the scrolling works fine.
- Close the banner.
- Scroll the page again.
Expected results: Scrolling is smooth, as is the case when there's no donation banner.
Actual results: Scrolling skips down some of the page when you pass a certain point (probably where the banner would be offscreen if it were there).
Workaround: Use the "disable fundraising banners" gadget.