overview: Unset visibleTarget on gesture end

The property describes the target visibility (that is, the visibility
that will be set after the ongoing transition), and is therefore updated
at the start of the transition rather than from hideDone().

The overview gesture currently misses resetting it at the end, so it
is only updated to the correct state the next time the overview is
entered.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3798

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1726>

js/ui/overview.js

@@ -396,6 +396,7 @@ var Overview = class {
         let onComplete;
         if (endProgress === 0) {
             this._shown = false;
+            this._visibleTarget = false;
             this.emit('hiding');
             Main.panel.style = 'transition-duration: %dms;'.format(duration);
             onComplete = () => this._hideDone();