From 078ed0b86a0d8e9f8457481cb739ea214195adce Mon Sep 17 00:00:00 2001
From: Maria Hutt <thetaPC@users.noreply.github.com>
Date: Mon, 23 Sep 2024 10:33:55 -0700
Subject: [PATCH] fix(segment): prevent flickering for scrollable on iOS
 (#29884)

Issue number: resolves #29523

---------

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issue. -->

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etc). Submit multiple pull requests if needed. -->

## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

The scrollable segment flickers on iOS physical devices or simulators
when the active button is near the edge of the screen. The jump is due
to the button being scrolled to the center and snaps back to the edge
since the button was scrolled past the container.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Switched to `scrollTo` provides for a smoother transition.
- Gave co author credit to the original reporter since they provided
part of the solution
- No new tests were created since functionality stays the same and
testing on Playwright would be impossible to recreate

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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1. Describe the impact and migration path for existing applications
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  2. Update the BREAKING.md file with the breaking change.
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## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

Dev build: 8.3.2-dev.11726779768.16e1f1d2

How to test:
1. Create a new app through any starter
2. Add a scrollable segment with at least 6 buttons (code snippet
example below)
3. Recommended to change the segment mode to `md` since it's easier to
see the flicker
4. Build the app and open it in an iOS or simulator (if more
instructions on how to do this is needed, reach out to me)
5. Click on the third button
6. Click on the first button
7. Notice the flicker
8. Click over to the third to last button
9. Click on either the last two buttons
10. Notice the flicker
11. Install the dev build
12. Verify the load does not flicker
13. Repeat steps 4 and 5
14. Verify the flicker is no longer there
15. Repeat steps 7 and 8
16. Verify the flicker is no longer there


```js
<ion-segment value="2" scrollable="true" mode="md">
  <ion-segment-button value="1">
    <ion-label>Button 1</ion-label>
  </ion-segment-button>
  <ion-segment-button value="2">
    <ion-label>Button 2</ion-label>
  </ion-segment-button>
  <ion-segment-button value="3">
    <ion-label>Button 3</ion-label>
  </ion-segment-button>
  <ion-segment-button value="4">
    <ion-label>Button 4</ion-label>
  </ion-segment-button>
  <ion-segment-button value="5">
    <ion-label>Button 5</ion-label>
  </ion-segment-button>
  <ion-segment-button value="6">
    <ion-label>Button 6</ion-label>
  </ion-segment-button>
</ion-segment>
```

---------

Co-authored-by: rostislavcz <58735164+rostislavcz@users.noreply.github.com>
---
 core/src/components/segment/segment.tsx | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/core/src/components/segment/segment.tsx b/core/src/components/segment/segment.tsx
index 4c425e18b2e..3ddf8645896 100644
--- a/core/src/components/segment/segment.tsx
+++ b/core/src/components/segment/segment.tsx
@@ -342,21 +342,35 @@ export class Segment implements ComponentInterface {
         const centeredX = activeButtonLeft - scrollContainerBox.width / 2 + activeButtonBox.width / 2;
 
         /**
-         * We intentionally use scrollBy here instead of scrollIntoView
+         * newScrollPosition is the absolute scroll position that the
+         * container needs to move to in order to center the active button.
+         * It is calculated by adding the current scroll position
+         * (scrollLeft) to the offset needed to center the button
+         * (centeredX).
+         */
+        const newScrollPosition = el.scrollLeft + centeredX;
+
+        /**
+         * We intentionally use scrollTo here instead of scrollIntoView
          * to avoid a WebKit bug where accelerated animations break
          * when using scrollIntoView. Using scrollIntoView will cause the
          * segment container to jump during the transition and then snap into place.
          * This is because scrollIntoView can potentially cause parent element
-         * containers to also scroll. scrollBy does not have this same behavior, so
+         * containers to also scroll. scrollTo does not have this same behavior, so
          * we use this API instead.
          *
+         * scrollTo is used instead of scrollBy because there is a
+         * Webkit bug that causes scrollBy to not work smoothly when
+         * the active button is near the edge of the scroll container.
+         * This leads to the buttons to jump around during the transition.
+         *
          * Note that if there is not enough scrolling space to center the element
          * within the scroll container, the browser will attempt
          * to center by as much as it can.
          */
-        el.scrollBy({
+        el.scrollTo({
           top: 0,
-          left: centeredX,
+          left: newScrollPosition,
           behavior: smoothScroll ? 'smooth' : 'instant',
         });
       }