From 69217407a80933bdaa052c299725d66b20750ec3 Mon Sep 17 00:00:00 2001
From: "mr. m" <91018726+mr-cheffy@users.noreply.github.com>
Date: Tue, 5 May 2026 11:37:46 +0200
Subject: [PATCH] gh-12699: Fixed switching windoww focus causing blinks
 (gh-13568)

---
 src/zen/sessionstore/ZenWindowSync.sys.mjs | 25 +++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/src/zen/sessionstore/ZenWindowSync.sys.mjs b/src/zen/sessionstore/ZenWindowSync.sys.mjs
index bc90b80c2..a8b0f69a1 100644
--- a/src/zen/sessionstore/ZenWindowSync.sys.mjs
+++ b/src/zen/sessionstore/ZenWindowSync.sys.mjs
@@ -97,11 +97,11 @@ class nsZenWindowSync {
   };
 
   /**
-   * Promise that resolves when the current docshell swap operation is finished.
+   * Promise|null that resolves when the current docshell swap operation is finished.
    * Used to avoid multiple simultaneous swap operations that could interfere with each other.
    * For example, when focusing a window AND selecting a tab at the same time.
    */
-  #docShellSwitchPromise = Promise.resolve();
+  #docShellSwitchPromise = null;
 
   /**
    * Map of sync handlers for different event types.
@@ -1485,18 +1485,29 @@ class nsZenWindowSync {
     ) {
       return;
     }
-    let promise = this.#docShellSwitchPromise;
+    if (this.#docShellSwitchPromise) {
+      return;
+    }
+    const onTabSelect = event => {
+      if (event.detail?.previousTab === event.target) {
+        return;
+      }
+      this.#lastSelectedTab = null;
+      this.on_TabSelect(event, { ignorePromise: true });
+    };
     this.#lastFocusedWindow = new WeakRef(window);
     this.#lastSelectedTab = new WeakRef(window.gBrowser.selectedTab);
+    window.addEventListener("TabSelect", onTabSelect, { once: true });
     // eslint-disable-next-line no-async-promise-executor
     this.#docShellSwitchPromise = new Promise(async resolve => {
-      await promise;
       await this.#onTabSwitchOrWindowFocus(window);
+      window.removeEventListener("TabSelect", onTabSelect);
       resolve();
+      this.#docShellSwitchPromise = null;
     });
   }
 
-  on_TabSelect(aEvent) {
+  on_TabSelect(aEvent, { ignorePromise = false } = {}) {
     const tab = aEvent.target;
     if (this.#lastSelectedTab?.deref() === tab) {
       return;
@@ -1504,11 +1515,15 @@ class nsZenWindowSync {
     this.#lastSelectedTab = new WeakRef(tab);
     const previousTab = aEvent.detail.previousTab;
     let promise = this.#docShellSwitchPromise;
+    if (promise && !ignorePromise) {
+      return;
+    }
     // eslint-disable-next-line no-async-promise-executor
     this.#docShellSwitchPromise = new Promise(async resolve => {
       await promise;
       await this.#onTabSwitchOrWindowFocus(tab.ownerGlobal, previousTab);
       resolve();
+      this.#docShellSwitchPromise = null;
     });
   }