From afbd927fc0e0f953e94d5a9c3791564d69192b5a Mon Sep 17 00:00:00 2001
From: egrs <egrs@users.noreply.github.com>
Date: Tue, 17 Feb 2026 15:58:38 +0100
Subject: [PATCH 1/5] support buffered option in PerformanceObserver.observe()

When buffered is true, deliver existing performance entries that match
the observer's interest immediately, per the Performance Observer spec.
---
 .../performance_observer.html                 | 28 +++++++++++++++++++
 src/browser/webapi/PerformanceObserver.zig    | 12 ++++++++
 2 files changed, 40 insertions(+)

diff --git a/src/browser/tests/performance_observer/performance_observer.html b/src/browser/tests/performance_observer/performance_observer.html
index 7b5b6c9e..efa90e80 100644
--- a/src/browser/tests/performance_observer/performance_observer.html
+++ b/src/browser/tests/performance_observer/performance_observer.html
@@ -69,3 +69,31 @@
 <script>
   testing.expectEqual(['mark', 'measure'], PerformanceObserver.supportedEntryTypes);
 </script>
+
+<script id="buffered_option">
+{
+  // Create marks BEFORE the observer
+  performance.mark("early1", { startTime: 1.0 });
+  performance.mark("early2", { startTime: 2.0 });
+
+  let receivedEntries = null;
+  const observer = new PerformanceObserver((list) => {
+    receivedEntries = list.getEntries();
+  });
+
+  // With buffered: true, existing marks should be delivered immediately
+  observer.observe({ type: "mark", buffered: true });
+
+  testing.eventually(() => {
+    testing.expectEqual(true, receivedEntries !== null);
+    // Should have received at least the 2 early marks
+    testing.expectEqual(true, receivedEntries.length >= 2);
+
+    const names = receivedEntries.map(e => e.name);
+    testing.expectEqual(true, names.indexOf("early1") >= 0);
+    testing.expectEqual(true, names.indexOf("early2") >= 0);
+
+    observer.disconnect();
+  });
+}
+</script>
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index ad130c97..cd6a672e 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -113,6 +113,18 @@ pub fn observe(
 
     // Update interests.
     self._interests = interests;
+
+    // Deliver existing entries if buffered option is set.
+    if (options.buffered) {
+        for (page.window._performance._entries.items) |entry| {
+            if (self.interested(entry)) {
+                try self._entries.append(page.arena, entry);
+            }
+        }
+        if (self.hasRecords()) {
+            try self.dispatch(page);
+        }
+    }
 }
 
 pub fn disconnect(self: *PerformanceObserver, page: *Page) void {

From df9779ec5985ae0afed5886624620234620fff87 Mon Sep 17 00:00:00 2001
From: egrs <egrs@users.noreply.github.com>
Date: Tue, 17 Feb 2026 16:12:20 +0100
Subject: [PATCH 2/5] restrict buffered option to type-based observation per
 spec

The buffered option is only valid with the type option, not entryTypes,
per the Performance Observer specification.
---
 src/browser/webapi/PerformanceObserver.zig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index cd6a672e..088128b1 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -115,7 +115,8 @@ pub fn observe(
     self._interests = interests;
 
     // Deliver existing entries if buffered option is set.
-    if (options.buffered) {
+    // Per spec, buffered is only valid with the type option, not entryTypes.
+    if (options.buffered and options.type != null) {
         for (page.window._performance._entries.items) |entry| {
             if (self.interested(entry)) {
                 try self._entries.append(page.arena, entry);

From ca9e2200da00f669b94e47aa00c1070b923ffa89 Mon Sep 17 00:00:00 2001
From: egrs <egrs@users.noreply.github.com>
Date: Tue, 17 Feb 2026 18:16:42 +0100
Subject: [PATCH 3/5] use async delivery for buffered performance observer
 entries

Per spec, buffered entries should be delivered via a queued task,
not synchronously. Extract scheduling logic into
schedulePerformanceObserverDelivery() and use it from both
notifyPerformanceObservers and the buffered observe path.
---
 src/browser/Page.zig                       | 5 +++++
 src/browser/webapi/PerformanceObserver.zig | 3 ++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 5940d2c5..c1350a0e 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -1243,6 +1243,11 @@ pub fn notifyPerformanceObservers(self: *Page, entry: *Performance.Entry) !void
         }
     }
 
+    try self.schedulePerformanceObserverDelivery();
+}
+
+/// Schedules async delivery of buffered performance observer records.
+pub fn schedulePerformanceObserverDelivery(self: *Page) !void {
     // Already scheduled.
     if (self._performance_delivery_scheduled) {
         return;
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index 088128b1..38f2a028 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -116,6 +116,7 @@ pub fn observe(
 
     // Deliver existing entries if buffered option is set.
     // Per spec, buffered is only valid with the type option, not entryTypes.
+    // Delivery is async via a queued task, not synchronous.
     if (options.buffered and options.type != null) {
         for (page.window._performance._entries.items) |entry| {
             if (self.interested(entry)) {
@@ -123,7 +124,7 @@ pub fn observe(
             }
         }
         if (self.hasRecords()) {
-            try self.dispatch(page);
+            try page.schedulePerformanceObserverDelivery();
         }
     }
 }

From 7c92e0e9ce7f387f1b05a98c83faeee72b8a9f98 Mon Sep 17 00:00:00 2001
From: egrs <egrs@users.noreply.github.com>
Date: Wed, 18 Feb 2026 07:14:12 +0100
Subject: [PATCH 4/5] address review: fix doc comment, skip buffered if already
 queued

---
 src/browser/Page.zig                       | 2 +-
 src/browser/webapi/PerformanceObserver.zig | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index c1350a0e..40b73618 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -1246,7 +1246,7 @@ pub fn notifyPerformanceObservers(self: *Page, entry: *Performance.Entry) !void
     try self.schedulePerformanceObserverDelivery();
 }
 
-/// Schedules async delivery of buffered performance observer records.
+/// Schedules async delivery of performance observer records.
 pub fn schedulePerformanceObserverDelivery(self: *Page) !void {
     // Already scheduled.
     if (self._performance_delivery_scheduled) {
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index 38f2a028..cfabaebb 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -117,7 +117,7 @@ pub fn observe(
     // Deliver existing entries if buffered option is set.
     // Per spec, buffered is only valid with the type option, not entryTypes.
     // Delivery is async via a queued task, not synchronous.
-    if (options.buffered and options.type != null) {
+    if (options.buffered and options.type != null and !self.hasRecords()) {
         for (page.window._performance._entries.items) |entry| {
             if (self.interested(entry)) {
                 try self._entries.append(page.arena, entry);

From f72a354066175adf28c77865f19800b0336852bd Mon Sep 17 00:00:00 2001
From: egrs <egrs@users.noreply.github.com>
Date: Wed, 18 Feb 2026 07:15:17 +0100
Subject: [PATCH 5/5] address review: clear marks before test, assert exactly 2

---
 .../performance_observer/performance_observer.html | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/browser/tests/performance_observer/performance_observer.html b/src/browser/tests/performance_observer/performance_observer.html
index efa90e80..a0797e15 100644
--- a/src/browser/tests/performance_observer/performance_observer.html
+++ b/src/browser/tests/performance_observer/performance_observer.html
@@ -72,6 +72,9 @@
 
 <script id="buffered_option">
 {
+  // Clear marks from previous tests so we get a precise count
+  performance.clearMarks();
+
   // Create marks BEFORE the observer
   performance.mark("early1", { startTime: 1.0 });
   performance.mark("early2", { startTime: 2.0 });
@@ -81,17 +84,14 @@
     receivedEntries = list.getEntries();
   });
 
-  // With buffered: true, existing marks should be delivered immediately
+  // With buffered: true, existing marks should be delivered
   observer.observe({ type: "mark", buffered: true });
 
   testing.eventually(() => {
     testing.expectEqual(true, receivedEntries !== null);
-    // Should have received at least the 2 early marks
-    testing.expectEqual(true, receivedEntries.length >= 2);
-
-    const names = receivedEntries.map(e => e.name);
-    testing.expectEqual(true, names.indexOf("early1") >= 0);
-    testing.expectEqual(true, names.indexOf("early2") >= 0);
+    testing.expectEqual(2, receivedEntries.length);
+    testing.expectEqual("early1", receivedEntries[0].name);
+    testing.expectEqual("early2", receivedEntries[1].name);
 
     observer.disconnect();
   });