I had this problem many times and i can't solve it till i restart my pc. App restart does not help as well.
When i'm working suddenly when i try to click any where in the editor the letter e keeps showing on.
Even when i try to restart i see a letter e on the corner of the editor window.
I can't really understand what is this problem exactly and how to avoid it in the future.
I use phpStorm Version 2021.2.1 and ubuntu 20.04