main: Remove transitions from child adjustments

When commit 4d963c432b introduced the global workspace adjustment,
it mostly copied the adjustment handling from the overview that it
set out to replace.

That includes cancelling ongoing transitions when the number of
workspaces changed. However that missed that transitions don't
happen on the main adjustment, but on the "child" adjustments
returned from `createWorkspacesAdjustment()`.

Address this by tracking all child adjustments, and cancel transitions
there as well when necessary. Use weak refs to not
interfere with garbage collection, in case an extension creates
its own child adjustment.

Fixes: 4d963c432b ("main: Introduce global workspaces adjustment")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7000
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2949>

js/ui/main.js

@@ -98,6 +98,7 @@ let _themeResource = null;
 let _oskResource = null;
 let _iconResource = null;
 let _workspacesAdjustment = null;
+let _workspaceAdjustmentRegistry = null;
 
 Gio._promisify(Gio.File.prototype, 'delete_async');
 Gio._promisify(Gio.File.prototype, 'touch_async');
@@ -445,6 +446,30 @@ function _loadDefaultStylesheet() {
     loadTheme();
 }
 
+class AdjustmentRegistry {
+    #count = 0;
+    #adjustments = new Map();
+    #registry = new FinalizationRegistry(key => {
+        this.#adjustments.delete(key);
+    });
+
+    register(adj) {
+        const key = this.#count++;
+        this.#adjustments.set(key, new WeakRef(adj));
+        this.#registry.register(adj, key);
+    }
+
+    forEach(callback) {
+        this.#adjustments.forEach((ref, key) => {
+            const adj = ref.deref();
+            if (adj)
+                callback(adj);
+            else
+                this.#adjustments.delete(key);
+        });
+    }
+}
+
 function _loadWorkspacesAdjustment() {
     const {workspaceManager} = global;
     const activeWorkspaceIndex = workspaceManager.get_active_workspace_index();
@@ -467,9 +492,12 @@ function _loadWorkspacesAdjustment() {
 
         // A workspace might have been inserted or removed before the active
         // one, causing the adjustment to go out of sync, so update the value
+        _workspaceAdjustmentRegistry.forEach(c => c.remove_transition('value'));
         _workspacesAdjustment.remove_transition('value');
         _workspacesAdjustment.value = newActiveIndex;
     });
+
+    _workspaceAdjustmentRegistry = new AdjustmentRegistry();
 }
 
 /**
@@ -498,6 +526,8 @@ export function createWorkspacesAdjustment(actor) {
     for (const [propName, flags] of properties)
         _workspacesAdjustment.bind_property(propName, adjustment, propName, flags);
 
+    _workspaceAdjustmentRegistry.register(adjustment);
+
     return adjustment;
 }