fix(segment): scroll to active segment-button on first load (#28276)

Issue number: resolves #28096

---------

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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. -->

When a segment button is clicked, the segment will auto-scroll to
position the newly active button fully in view. However, this behavior
does not occur on first load. This means that when a segment is
initialized with a `value` corresponding to an off-screen button, the
button will remain off-screen.

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

The same auto-scroll behavior from button click now also occurs on
component load.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this introduces a breaking change, please describe the impact
and migration path for existing applications below. -->


## Other information

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

core/src/components/segment/segment.tsx

@@ -1,6 +1,7 @@
 import type { ComponentInterface, EventEmitter } from '@stencil/core';
 import { Component, Element, Event, Host, Listen, Prop, State, Watch, h, writeTask } from '@stencil/core';
 import type { Gesture, GestureDetail } from '@utils/gesture';
+import { raf } from '@utils/helpers';
 import { isRTL } from '@utils/rtl';
 import { createColorClasses, hostContext } from '@utils/theme';
 
@@ -83,31 +84,7 @@ export class Segment implements ComponentInterface {
      * Used by `ion-segment-button` to determine if the button should be checked.
      */
     this.ionSelect.emit({ value });
-
+    this.scrollActiveButtonIntoView();
-    if (this.scrollable) {
-      const buttons = this.getButtons();
-      const activeButton = buttons.find((button) => button.value === value);
-      if (activeButton !== undefined) {
-        /**
-         * Scrollable segment buttons should be
-         * centered within the view including
-         * buttons that are partially offscreen.
-         */
-        activeButton.scrollIntoView({
-          behavior: 'smooth',
-          inline: 'center',
-
-          /**
-           * Segment should scroll on the
-           * horizontal axis. `block: 'nearest'`
-           * ensures that the vertical axis
-           * does not scroll if the segment
-           * as a whole is already in view.
-           */
-          block: 'nearest',
-        });
-      }
-    }
   }
 
   /**
@@ -163,6 +140,14 @@ export class Segment implements ComponentInterface {
   async componentDidLoad() {
     this.setCheckedClasses();
 
+    /**
+     * We need to wait for the buttons to all be rendered
+     * before we can scroll.
+     */
+    raf(() => {
+      this.scrollActiveButtonIntoView();
+    });
+
     this.gesture = (await import('../../utils/gesture')).createGesture({
       el: this.el,
       gestureName: 'segment',
@@ -320,6 +305,35 @@ export class Segment implements ComponentInterface {
     }
   }
 
+  private scrollActiveButtonIntoView() {
+    const { scrollable, value } = this;
+
+    if (scrollable) {
+      const buttons = this.getButtons();
+      const activeButton = buttons.find((button) => button.value === value);
+      if (activeButton !== undefined) {
+        /**
+         * Scrollable segment buttons should be
+         * centered within the view including
+         * buttons that are partially offscreen.
+         */
+        activeButton.scrollIntoView({
+          behavior: 'smooth',
+          inline: 'center',
+
+          /**
+           * Segment should scroll on the
+           * horizontal axis. `block: 'nearest'`
+           * ensures that the vertical axis
+           * does not scroll if the segment
+           * as a whole is already in view.
+           */
+          block: 'nearest',
+        });
+      }
+    }
+  }
+
   private setNextIndex(detail: GestureDetail, isEnd = false) {
     const rtl = isRTL(this.el);
     const activated = this.activated;

core/src/components/segment/test/scrollable/segment.e2e.ts

@@ -2,7 +2,7 @@ import { expect } from '@playwright/test';
 import { configs, test } from '@utils/test/playwright';
 
 configs().forEach(({ title, screenshot, config }) => {
-  test.describe(title('segment: scrollable'), () => {
+  test.describe(title('segment: scrollable (rendering)'), () => {
     test('should not have visual regressions', async ({ page }) => {
       await page.setContent(
         `
@@ -45,3 +45,47 @@ configs().forEach(({ title, screenshot, config }) => {
     });
   });
 });
+
+configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, config }) => {
+  test.describe(title('segment: scrollable (functionality)'), () => {
+    test('should scroll active button into view when value is already set', async ({ page }) => {
+      await page.setContent(
+        `
+          <ion-segment scrollable="true" value="8">
+            <ion-segment-button value="1">
+              <ion-label>First</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="2">
+              <ion-label>Second</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="3">
+              <ion-label>Third</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="4">
+              <ion-label>Fourth</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="5">
+              <ion-label>Fifth</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="6">
+              <ion-label>Sixth</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="7">
+              <ion-label>Seventh</ion-label>
+            </ion-segment-button>
+            <ion-segment-button id="activeButton" value="8">
+              <ion-label>Eighth</ion-label>
+            </ion-segment-button>
+            <ion-segment-button value="9">
+              <ion-label>Ninth</ion-label>
+            </ion-segment-button>
+          </ion-segment>
+        `,
+        config
+      );
+
+      const activeButton = page.locator('#activeButton');
+      await expect(activeButton).toBeInViewport();
+    });
+  });
+});