Audacity does three rather counter-intuitive things relating to track select/focus.
(a) I display a stereo track and a label track, and have the stereo track selected and in focus (i.e. the yellow box is around the stereo track, the header of the stereo track (but not of the label track) is blue-grey, and no part of the track is dark grey. (Sorry this description is clumsy.)
(b) I click on the timer track to start playing, I press "[" and later "]" to select a portion of music by ear, and hit SPACE to stop play. Then I press "CTL-B" and type in a name for the label, and hit ENTER.
Now I attempt to repeat the process from (a), to select and label the next bit of music, but Audacity fails me! It does something very odd indeed, ending up by labelling a point in my track with the label "] " and refusing to stop playing!
After experimenting, I realized this is because, after I hit ENTER, Audacity has left the focus (yellow box) around the label track - even though I am now playing the stereo track and clearly MY attention is focussed there. I have to manually refocus Audacity on the stereo track, using SHIFT and the up/down keys.
(2) If I complete step B, with my labelled portion of music selected (darkened), and then attempt to use Equalization (e.g. to amplify the music portion), Audacity will either do nothing or apply equalization to the whole track!
Again, experiment shows that this is because I have left the focus on the stereo track. But clearly MY focus is on the music!
(3) If I start step (b) with the focus (yellow box) on the stereo track but with the track NOT selected (its header is almost white), Audacity will allow me to "select" a portion of music with "[" and "]", and show this section darkened in the timer track only. If I currently have no other track displayed, there is no way to apply this selection to the stereo track, so that I may edit it!
I have to add a label track, then use SHIFT up/down keys, in order to apply this selection to the stereo track. Weird, eh?
Forgive me taking up everyone's time. I hope the workarounds may help anyone stuck like me, and would like to suggest modifications to make Audacity guess what the user is focussed on, when it's obvious.