From 6c0172f04991dcae66d3d0a5b3fa5fcc842d4e53 Mon Sep 17 00:00:00 2001
From: Greg Stoll <gstoll@mozilla.com>
Date: Thu, 21 Nov 2024 12:34:15 +0000
Subject: [PATCH] Bug 1927609 - correctly set process priority when rapidly
 switching tabs r=tabbrowser-reviewers,dao

Differential Revision: https://phabricator.services.mozilla.com/D229696
---
 .../tabbrowser/AsyncTabSwitcher.sys.mjs       | 13 ++++++-
 .../tests/browser_ProcessPriorityManager.js   | 39 +++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/browser/components/tabbrowser/AsyncTabSwitcher.sys.mjs b/browser/components/tabbrowser/AsyncTabSwitcher.sys.mjs
index c8e246184314..234eae789110 100644
--- a/browser/components/tabbrowser/AsyncTabSwitcher.sys.mjs
+++ b/browser/components/tabbrowser/AsyncTabSwitcher.sys.mjs
@@ -928,8 +928,8 @@ export class AsyncTabSwitcher {
   }
 
   /**
-   * Check if the browser should be deactivated. If the browser is a print preivew or
-   * PiP browser then we won't deactive it.
+   * Check if the browser should be deactivated. If the browser is a print preview or
+   * PiP browser then we won't deactivate it.
    * @param browser The browser to check if it should be deactivated
    * @returns false if a print preview or PiP browser else true
    */
@@ -1061,6 +1061,15 @@ export class AsyncTabSwitcher {
     this.requestedTab = tab;
     if (tabState == this.STATE_LOADED) {
       this.maybeVisibleTabs.clear();
+      // We're switching to a tab that is still loaded.
+      // Make sure its priority is correct as it may
+      // have been deprioritized when it was switched
+      // away from (bug 1927609)
+      let browser = tab.linkedBrowser;
+      let remoteTab = browser.frameLoader?.remoteTab;
+      if (remoteTab) {
+        remoteTab.priorityHint = true;
+      }
     }
 
     tab.linkedBrowser.setAttribute("primary", "true");
diff --git a/dom/ipc/tests/browser_ProcessPriorityManager.js b/dom/ipc/tests/browser_ProcessPriorityManager.js
index df0f80a93f73..f4e2b5e2c577 100644
--- a/dom/ipc/tests/browser_ProcessPriorityManager.js
+++ b/dom/ipc/tests/browser_ProcessPriorityManager.js
@@ -993,3 +993,42 @@ add_task(async function test_tab_moved_to_new_window() {
     }
   );
 });
+
+/**
+ * Test that if a tab is quickly switched away from and back to, it ends up at
+ * PROCESS_PRIORITY_FOREGROUND.
+ * See bug 1927609.
+ */
+add_task(async function test_tab_quickly_switched() {
+  let originalTab = gBrowser.selectedTab;
+  let origtabID = browsingContextChildID(
+    originalTab.linkedBrowser.browsingContext
+  );
+
+  await BrowserTestUtils.withNewTab(
+    "https://example.com/browser/dom/ipc/tests/file_dummy.html",
+    async browser => {
+      let tab = gBrowser.getTabForBrowser(browser);
+      let tabID = browsingContextChildID(tab.linkedBrowser.browsingContext);
+
+      // Don't use BrowserTestUtils.switchTab() because things have settled
+      // by the time it's done, which doesn't expose this bug.
+      gBrowser.selectedTab = originalTab;
+      gBrowser.selectedTab = tab;
+      await new Promise(resolve =>
+        // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+        setTimeout(resolve, WAIT_FOR_CHANGE_TIME_MS)
+      );
+      Assert.equal(
+        gTabPriorityWatcher.currentPriority(tabID),
+        PROCESS_PRIORITY_FOREGROUND,
+        "Active tab should be foreground priority"
+      );
+      Assert.equal(
+        gTabPriorityWatcher.currentPriority(origtabID),
+        PROCESS_PRIORITY_BACKGROUND,
+        "Inactive tab should be background priority"
+      );
+    }
+  );
+});