The Main tab was a fixed 50/50 grid, but a map and a cluster list want
very different widths and which one deserves the room changes with what
the operator is doing.
The share is clamped to 15..85: a pane can be made small but never
dragged out of existence, because recovering from that would mean
grabbing a divider no longer on screen. Double-click restores even.
The drag uses pointer capture on the divider — without it Leaflet
swallows the moves as soon as the cursor crosses the map. Persisted
through writeUiPref, so it travels with data/ like the other UI prefs.
rouggy
a83acb0f9a