I have been using LiberOffice for the last week and without fail I will pause after writing for 15 minutes or so. When I resume writing, what I’ve written will be highlighted and if I am not paying EXTREME attention with my next keystroke what I have written will be deleted. There is a similar situation when the cursor “floats” to another passage I’ve written and when I begin to type again my new words appear in another section that I’ve just written
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As a wild guess, are you on Windows and do you have set in the File Explorer Options dialogue Single-click to open an item (point to select)?
I had a couple of mice that did that, especially under LED or fluorescent light, I eventually learned and bought a decent mouse.
I guess “highlighted” is selected, so what you type next will replace the selected area. But you should be able to undo this (from menu or with key Ctrl-z) after deletion.
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Actually I’ve never seen this, as I usually lock my screen…