Page chooser button should remain highlighted while the dropdown is open

[Example profile](https://deploy-preview-5182--perf-html.netlify.app/public/1pv36v8pgyn1sg1pmsry542d5vn3yd1wtygcd8r/calltree/?globalTrackOrder=e0wd&hiddenGlobalTracks=1wb&hiddenLocalTracksByPid=1143648-0w6~1143767-0~1143926-0~1144193-0~1144031-0~1144736-0~1143930-0~1144584-0~1143912-0~1144366-0~1143805-01&thread=k&timelineType=category&v=10)

Steps to reproduce:
1. Click on "Full Range (3.4s)"
2. While the dropdown is open, move your mouse away from the button that opened it.

Expected results:
As long as the dropdown is open, the button's hover effect should remain visible.

Actual results:
The button's hover effect is lost.

Actually maybe we should have an even darker highlight, as part of a "pressed"/"open" state for the button.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-813)