library.browse (engine and the UI's background read) switched the source to the
folder without touching the selection, so the previous view's photo stayed
active: Photo Grid marked nothing, and Detail, the histogram and the info
showed a photo the folder doesn't hold. Browsing now follows library.source's
rule through a shared Session::keep_active_in_view: the active photo stays when
the folder shows it, else the folder's first photo becomes active (also as the
background read's batches arrive).
Jason Baldwin
70a12af9a2