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There are also issues on mobile devices when entering large amounts of text, as the editor's scroll jumps according to some unknown algorithm, and very often the editor window is not positioned where the cursor is, but somewhere much closer to the beginning. This makes deleting text very inconvenient.
Although some of the mobile issues seem to be caused by the standard input method in the OS (regarding the previous issue with the cursor suddenly jumping – I discovered it not only on the forum but also later in a completely different text editor, so there is no connection here). However, for some reason, this was less noticeable in the old editor. Perhaps it is possible to disable standard input functions but somehow retain the pop-up menus for copying and pasting? I don't even know...
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And there is another question: have you tried using the editor that is used for news articles for the forum? It does look a bit clunky compared to the current message editor, but it also supports HTML. I don't remember how it worked on mobile, but on PC, I don't recall noticing any particular problems with it.