[Sidebar view] Re-add mouse area for handling clicks on subcategory header

Now, even though there is a real button you can click on, if you happen to click on any other part of the subcategory page header, it also goes back, just like it used to. This is kind of hidden, but if we don't do this, we will surely get complaints from people who had gotten used to the nice big click area we had before.

cc @teams/vdg