fix(segment-button): update checked state on render (#27183)

This backports https://github.com/ionic-team/ionic-framework/pull/26970
to v6.

resolves https://github.com/ionic-team/ionic-framework/issues/26830

Co-authored-by: Sean Perkins <sean@ionic.io>

core/src/components/segment-button/segment-button.tsx

@@ -1,5 +1,5 @@
 import type { ComponentInterface } from '@stencil/core';
-import { Component, Element, Host, Prop, Method, State, forceUpdate, h } from '@stencil/core';
+import { Component, Element, Host, Prop, Method, State, Watch, forceUpdate, h } from '@stencil/core';
 
 import { getIonMode } from '../../global/ionic-global';
 import type { SegmentButtonLayout } from '../../interface';
@@ -53,6 +53,10 @@ export class SegmentButton implements ComponentInterface, ButtonInterface {
    * The value of the segment button.
    */
   @Prop() value: string = 'ion-sb-' + ids++;
+  @Watch('value')
+  valueChanged() {
+    this.updateState();
+  }
 
   connectedCallback() {
     const segmentEl = (this.segmentEl = this.el.closest('ion-segment'));

core/src/components/segment/test/segment.spec.ts

@@ -0,0 +1,24 @@
+import { newSpecPage } from '@stencil/core/testing';
+
+import { SegmentButton } from '../../segment-button/segment-button';
+import { Segment } from '../segment';
+
+it('should set checked state when value is set asynchronously', async () => {
+  const page = await newSpecPage({
+    components: [Segment, SegmentButton],
+    html: `
+      <ion-segment value="first">
+        <ion-segment-button>First</ion-segment-button>
+      </ion-segment>
+    `,
+  });
+
+  const segmentButton = page.root.querySelector('ion-segment-button');
+
+  expect(segmentButton.classList.contains('segment-button-checked')).toBe(false);
+
+  segmentButton.value = 'first';
+  await page.waitForChanges();
+
+  expect(segmentButton.classList.contains('segment-button-checked')).toBe(true);
+});