gh-14905: Inherit context IDs when syncing tabs (gh-14908)

src/zen/sessionstore/ZenWindowSync.sys.mjs

@@ -1373,6 +1373,7 @@ class nsZenWindowSync {
       const newTab = win.gBrowser.addTrustedTab("about:blank", {
         animate: true,
         createLazyBrowser: true,
+        userContextId: tab.userContextId,
         _forZenEmptyTab: tab.hasAttribute("zen-empty-tab"),
       });
       newTab.id = tab.id;

src/zen/tests/window_sync/browser_sync_tab_open.js

@@ -16,3 +16,41 @@ add_task(async function test_SimpleTabOpen() {
     );
   });
 });
+
+add_task(async function test_TabOpenInContainer() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["privacy.userContext.enabled", true]],
+  });
+  let newTab = null;
+  await withNewSyncedWindow(async win => {
+    await runSyncAction(
+      () => {
+        newTab = gBrowser.addTrustedTab("https://example.com/", {
+          inBackground: true,
+          userContextId: 1,
+        });
+      },
+      async () => {
+        Assert.equal(
+          newTab.userContextId,
+          1,
+          "The opened tab should keep its container"
+        );
+        const otherTab = gZenWindowSync.getItemFromWindow(win, newTab.id);
+        Assert.ok(
+          otherTab,
+          "The opened tab should be found in the synced window"
+        );
+        Assert.equal(
+          otherTab.userContextId,
+          newTab.userContextId,
+          "The synced tab should inherit the original tab's container"
+        );
+      },
+      "TabOpen"
+    );
+  });
+  let tabClosing = BrowserTestUtils.waitForTabClosing(newTab);
+  BrowserTestUtils.removeTab(newTab);
+  await tabClosing;
+});