I'm copy/pasting from https://www.mediawiki.org/wiki/Topic:Tn9sxkrou2a264xd .
An image resized to full horizontal width, using the visual editor goes overflows and may obscure the text in the sidebar.
Go to an article with an image, e.g. VisualEditor/Design/User testing and enter edit mode
Click on File:July31 toolbar.png
Click Advanced settings
Click make full size
Click apply changes
Click publish changes
Click review changes
The image doesn't on top of the sidebar. A scroll bar can be use to see the rest of it.
The image goes beyond the preview section and is also visible in the sidebar".
TheDJ confirms and adds, "Might want to have at least overflow:hidden on .ve-ui-diffElement-content" .
Our IP friend adds, "Yes, this also happens full width (100%) "<pre>" blocks, and overflow hidden wouldn't work too well because it would hide some entered content, unless maybe a scroll bar is added to the separate preview area."