Steps to reproduce:

1. Go to main search page (https://en.wikipedia.org/w/index.php?search=, https://commons.wikimedia.org/w/index.php?search=) and type in the search box. Alternatively, get there by typing gobbledygooksdfdsfdsfds into the ever-present quick-search box.
2. Now try to move your cursor to edit somewhere earlier in the string by clicking with your mouse.
3. this is possibly related to the appearance of an auto-complete dropdown that now appears when you type slowly from the beginning.

Using Chrome Version 77.0.3865.120, on macos. Applied both logged in, logged out, incognito. Noticed it for the first time: October 18th.