Steps to reproduce, from https://www.mediawiki.org/wiki/Topic:Vxjjwid5sympsajv:
- Insert image in source editor
- Switch to VisualEditor. Image should be visible.
- Click on the image
- A tooltip appears. Click its Edit button.
- There are two tabs on the top: General and Advanced. Click on Advanced.
- Scroll down to Image size.
- There are two options: Default and Custom. Click the latter.
- Double-click the first number to highlight it
- Try entering a new number
Upon the last step, it switched back to Default again, apparently because the one-digit width pixel number is initially too low. But I intended to type in a three-digit width.