This affects at least Firefox. When a message has an inline codeblock containing a long string of tabs this causes the chat window to have a horizontal scrollbar and to scroll spontaneously left and right as new messages appear.
This does not happen when messages use fixed font to create code blocks.
Since the chat sandbox was difficult to test in due to already having the problem, I've tested this in a new room and confirmed that the problem does not happen when pasting in a long string of tabs after 4 spaces (to create a code block). It only happens when enclosing a long string of tabs in backticks to create