fix: rebuild allAutoStickyStates with nested iteration instead of flat() (#3840)

Array.prototype.flat() only flattens arrays, not Sets.
Since autoStickyStates is a Map<string, Set<string>>, spreading
its values and calling flat() resulted in Set objects themselves
being stored in allAutoStickyStates instead of their individual
state strings.

Author: tkng

webextensions/common/TreeItem.js

@@ -797,9 +797,12 @@ export class TreeItem {
     else
       TreeItem.autoStickyStates.delete(providerId);
 
-    TreeItem.allAutoStickyStates = new Set([
-      ...TreeItem.autoStickyStates.values(),
-    ].flat());
+    TreeItem.allAutoStickyStates = new Set();
+    for (const states of TreeItem.autoStickyStates.values()) {
+      for (const state of states) {
+        TreeItem.allAutoStickyStates.add(state);
+      }
+    }
 
     TreeItem.updateCanBecomeStickyTabsIndex(TabsStore.getCurrentWindowId());