Just to note, I can supply more details if this is still undetected / to be traced:
I opened a folder containing 16 subfolders, with 1 text file each (around 300.000 characters total, 10-20.000 per text file) in the binder by clicking on it. The text is displayed as it should in the editor window.
If I now enter Compose mode (black background), and start editing and proofreading, and edit a word – either delete or change or insert – often, but not always the screen blackens out. It looks as if the cursor would have jumped some lines higher into a black area.
When I exit Compose, all is as it should be, though the text is displayed at the text file’s starting position, not at the current position of the edit.
System used is a not-too-fast, but just some years old acer TravelMate notebook. Win 10, current version.