Hello,
This highlighting is representing something different than the track display, which we may not be very clear about. What the highlighting in this case is showing is the transcript you selected from your search results.
You probably got here by searching for TP53 . If you click the first search result from that link, you'll see the third transcript highlighted (same as the picture you provided). You can try clicking others, and you'll see that it will highlight different ones.
If you look at the search results again, you'll notice some of these are in bold. These represent the canonical transcript for that gene, in this case it's the first hit.
So the bold hits in the search results are canonical, and then the highlighted item in hgTracks corresponds to your search selection.
If you have any additional questions, you can reach out to our help desk at genome@soe.ucsc.edu. We check Biostars every now and again, but your question may go unanswered for some time. If you post a question here, using the "ucsc" helps to put it on our radar.
Lou
UCSC Genome Browser
Possibly canonical transcript? - you may want to confirm via the UCSC directly (and post the answer back here)