Unreal shows a yellow line around active viewport but the current panel receiving input is not always clear as it is with other programs.
If you’re using sequencer and using the arrows to step forward and back frames in the sequence and then touch anything in the viewport the next time you use an arrow key the entire view will shift. Clicking on the sequencer timeline will re-focus there but there is no indicator that is the case.
Same applies to some other panels as well.