Add changelog for EventTimingTapStopScrollNoInteractionId

This CL adds metric changelog for an INP feature
EventTimingTapStopScrollNoInteractionId.

Bug: 335238186
Change-Id: I0a009eb869a36a83cd5184adfbc1ae6810919479
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5973135
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Reviewed-by: Michal Mocny <mmocny@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1375923}

docs/speed/metrics_changelog/2024_10_inp.md

@@ -54,3 +54,36 @@ desktop platforms only, which is also expected.
 
 Launch was the week of Oct 21, 2024, when m130 was in stable, but a limited
 number of early adopters might have seen this change as early as m129.
+
+## Enable EventTimingTapStopScrollNoInteractionId by default
+
+This experiment excludes taps and clicks which stop a fling scroll from INP measurement.
+
+During an active fling, a tap or a click can make it stop, but these events are
+not the same as a normal tap or click. For example, the pointerdown and
+pointerup events are fired but no click events are fired, default user agent
+actions aren’t triggered on the elements (i.e. links and buttons don't get
+clicked). More importantly, these interactions occur in the middle of an active
+scroll and the first visual feedback that the user receives is that the scroll
+stops. That "scroll stop" animation frame is entirely driven by the compositor
+and is not the next paint as measured by INP. It is also problematic to measure
+the events with the scroll animation.
+
+Note: this experiment was first landed in Chrome 129:
+[Don't set interaction ids to the input events when the scroll is active](https://chromium-review.googlesource.com/c/chromium/src/+/5579814), and was slowly rolled out.
+
+### How does this affect a site's metrics?
+
+This affects the INP metrics mainly on Android which uses touch screens.
+
+On Android (where touch driven scroll fling is a common gesture), we observed a
+slight decrease in the number of INP events recorded, due to a small number of
+page loads having only fling stops as the only interaction. These page loads no
+longer report INP. We also observed a slight decrease in INP at the 95th and 99th
+percentiles. Some sites may see a small reduction in the number of page loads
+where INP is recorded, and a slight decrease in INP overall.
+
+### When were users affected?
+
+Launch was the week of Oct 21, 2024, when m130 was in stable, but a limited
+number of early adopters might have seen this change as early as m129.

docs/speed/metrics_changelog/inp.md

@@ -3,6 +3,7 @@
 This is a list of changes to [Interaction to Next Paint](https://web.dev/inp).
 
 * Chrome 130
+  * Launch feature: [Enable EventTimingTapStopScrollNoInteractionId by default](2024_10_inp.md)
   * Launch feature: [Enable EventTimingHandleKeyboardEventSimulatedClick by default](2024_10_inp.md)
   * Launch feature: [Enable ReportEventTimingAtVisibilityChange by default](2024_10_inp.md)
 * Chrome 129