This is unusual. I log into eBay in the Brave browser. Within eBay I click a message and then click to reply. A window appears for me to type my reply. With each character I type it loses focus … meaning that if I wanted to type thank you I would type T, then click back to the window, h, then click back to the window, a, then click back to the window and so on
I tested this on 3 different computers (Windows 10 & Mac OSX) and got the same result. I then tested it on Safari on MAc OSX and it worked!