I also run it in Chrome/XFCE and this has been a bug since the beginning of v4. I ended up simply remapping all actions based on F5. I use F7/F8 to run plan/query.
I noticed that from time to time query edit box out of the blue loses focus e.g. as I type SELECT <space> it stops typing and I discover that focus is no longer on that input area. This may be related to F5 issue. When the element that is supposed to listen to events is no longer receiving events the keyboard keys it intercepts will be processed by the browser with its default handlers.
This is just a speculation but it would explain both issues.
Loss of focus could be also caused by Chrome extensions, spellchecker, etc though this never happened to me in e.g. gmail or google docs or any other web app so I am inclined to think this is a pgadmin bug.
I use F5 to execute queries. In previous versions of pgAdmin4 if F5 was going to reload the page a warning message allowed you to cancel the page reload, and thus save your work, reduce (slightly) frustration, etc. But in 3.5 that seems to be gone. I'm running current Chrome in Ubuntu.