I'm using a wide screen. Starting with a narrow browser window, and gradually pulling the window out to make it wider, this is what happens, in order:
1. The text initially flows around the infobox, contents list and images, which all fit within the window boundaries (expected)
2. The white margins get gradually wider (expected)
3. The infobox and the right images suddenly pop out of the text area into the right margin. The text area becomes //narrower// (not expected)
4. Finally, the contents list and left images suddenly pop out of the text area into the left margin. The text area becomes narrower again (not expected), and at the same time the font size of the main text increases significantly (not expected).
On my screen, with a typical article [[Selwyn College, Cambridge]], at level 1 the text area has a width of 23 cm, with 13 words fitting into the first line. By level 4, there is a much narrower text area of only 9 cm, and with the larger font only 3 words will fit into the first line. This is surely not intended behaviour.
In case it's relevant, I'm using Firefox 81.0 on a Mac.