@Pbsouthwood has diligently attempted to figure out an extremely annoying bug (or two?). The main symptom is that when certain common actions are taken, the cursor leaves the paragraph that you're typing and jumps to the top of the editing window. These actions include:
- using the Shift key
- pasting something into a message
You can see his efforts and real-time documentation of the results beginning at https://en.wikipedia.org/wiki/User_talk:Whatamidoing_(WMF)#c-Pbsouthwood-20220825173600-Problem_with_add_topic_tool
This appears in Firefox on both Windows 10 and Windows 11. I have occasionally seen something similar in Firefox on my Mac, but much less frequently and only while pasting.