Pearli at mw.org: (edited)
Let's suppose you want to edit the site https://en.wikipedia.org/wiki/Overwatch_(video_game)?veaction=editsource on english wikipedia. If you try to mark a big text passage like the first two sections from bottom up to top, the browser always jumps to the cursor position where you started. It is not important which text passages you want to mark. It happens to all, with and without wiki-syntax.
The 2nd thing, top to down, is a bit harder, it didn't jump to the bottom of the page, it is hopping large, large text passages down. You can't really do accuracy work if this happens. It is a bit frustrating and i switched to old wikitext editor to "solve" the problem and switched back after I finished my work.