Let a click on the separator wander a little before it counts as a drag (#34580)

The separator has to tell a click apart from a drag, because dragging it
also ends in a click, and it did so by treating any pointer movement at all
between press and release as a drag. A pointer rarely holds perfectly
still, least of all on a trackpad, so clicking the separator to open the
room list often did nothing and had to be tried again.

Movement is now measured from where the pointer went down and only counts
as a drag past a few pixels, which means the handlers need the pointer
position and the separator passes its events through to get it. Movement
with nothing held down is ignored too: those events fire on hover, and one
of them used to spend the click that came after it.

Tests: a click that wanders a couple of pixels still opens the panel, as
does one that follows moving across the separator, while a real drag is
still no click.

Co-authored-by: R Midhun Suresh <hi@midhun.dev>

packages/shared-components/src/resize/separator/SeparatorView.tsx

@@ -23,14 +23,16 @@ export interface SeparatorViewActions {
     onPointerUp: () => void;
 
     /**
-     * onPointerMove handler for separator.
+     * onPointerMove handler for separator. Takes the event so that how far the pointer has travelled
+     * since it went down can be measured.
      */
-    onPointerMove: () => void;
+    onPointerMove: (event: React.PointerEvent) => void;
 
     /**
-     * onPointerDown handler for separator.
+     * onPointerDown handler for separator. Takes the event so that where the pointer went down can be
+     * measured from.
      */
-    onPointerDown: () => void;
+    onPointerDown: (event: React.PointerEvent) => void;
 
     /**
      * onDoubleClick handler for the separator.