Common issue when trying to highlight text is that the web browser can cause highlight to be toggled on and require you to click inside the overlay Web Browser to toggle off.

If the mouse is placed behind the text input area, you write in reverse, nice trick that is useless generally, place mouse pointer to spot that intersects the text input area vertically can cause writing issues, you expect to be writing, but now you just deleted bunch of the text and so on.

It is very hard to acclimate to, because it is unique to Steam Overlay Web Browser only, it doesn't happen on any other Web Browser or text editing program.