The mounted filter panel, or null when none is.
The filter configuration the panel was rendered from.
What asked for the pass: "filter" (the default) when the filter itself was
applied, "data" when the same filter is replayed because a layer's store changed.
Reads the panel DOM, expands taxonomy selections, applies the filter to every source, says in the panel when no feature was kept (
writePanelMatchStatus) and notifies listeners (geoleaf:filters:applied, with the reason of the pass). Single entry point for Apply / control changes / reset, and for the pass an active filter replays when a layer is written; the permalink restore goes throughGeoLeaf.Filter.applyFilter, which does the same from a serialised state.