fix(datetime): ensure datetime is shown when intersection observer fails to report visibility (#30793)

Issue number: resolves #30706

---------

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issue. -->

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## What is the current behavior?

Due to some recent unknown changes, the intersection observer for date
time no longer reliably fires, especially in mobile views.

## What is the new behavior?

In this PR, we're adding a visibility check after everything has had a
chance to render to make sure we're setting up properly even if the
intersection observer has failed to trigger for some reason.

## Does this introduce a breaking change?

- [ ] Yes
- [X] No

## Other information

Since the intersection observer is being set up after a `raf`, it's
possible something got introduced to make the initial setup slower for
some reason, causing timing issues. I think we should do a more thorough
investigation into the cause of this problem when we have more time.

This PR also adds tests to verify the new fallback works properly.

Current dev build:
```
8.7.10-dev.11763478209.1d9c4cd8
```

---------

Co-authored-by: Brandy Smith <brandyscarney@users.noreply.github.com>

core/src/components/datetime-button/test/overlays/datetime-button.e2e.ts

@@ -1,7 +1,7 @@
-import { expect } from '@playwright/test';
 import type { Locator } from '@playwright/test';
-import { configs, test } from '@utils/test/playwright';
+import { expect } from '@playwright/test';
 import type { EventSpy } from '@utils/test/playwright';
+import { configs, test } from '@utils/test/playwright';
 
 /**
  * This behavior does not vary across directions.
@@ -176,5 +176,34 @@ configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, config }) => {
       await ionModalDidPresent.next();
       await expect(datetime).toBeVisible();
     });
+    test('should set datetime ready state and keep calendar interactive when reopening modal', async ({
+      page,
+    }, testInfo) => {
+      testInfo.annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/30706',
+      });
+
+      const openAndInteract = async () => {
+        await page.click('#date-button');
+        await ionModalDidPresent.next();
+
+        await page.locator('ion-datetime.datetime-ready').waitFor();
+
+        const calendarBody = datetime.locator('.calendar-body');
+        await expect(calendarBody).toBeVisible();
+      };
+
+      await openAndInteract();
+
+      const firstEnabledDay = datetime.locator('.calendar-day:not([disabled])').first();
+      await firstEnabledDay.click();
+      await page.waitForChanges();
+
+      await modal.evaluate((el: HTMLIonModalElement) => el.dismiss());
+      await ionModalDidDismiss.next();
+
+      await openAndInteract();
+    });
   });
 });

core/src/components/datetime/datetime.tsx

@@ -1101,6 +1101,32 @@ export class Datetime implements ComponentInterface {
     this.initializeKeyboardListeners();
   }
 
+  /**
+   * TODO(FW-6931): Remove this fallback upon solving the root cause
+   * Fallback to ensure the datetime becomes ready even if
+   * IntersectionObserver never reports it as intersecting.
+   *
+   * This is primarily used in environments where the observer
+   * might not fire as expected, such as when running under
+   * synthetic tests that stub IntersectionObserver.
+   */
+  private ensureReadyIfVisible = () => {
+    if (this.el.classList.contains('datetime-ready')) {
+      return;
+    }
+
+    const rect = this.el.getBoundingClientRect();
+    if (rect.width === 0 || rect.height === 0) {
+      return;
+    }
+
+    this.initializeListeners();
+
+    writeTask(() => {
+      this.el.classList.add('datetime-ready');
+    });
+  };
+
   componentDidLoad() {
     const { el, intersectionTrackerRef } = this;
 
@@ -1141,6 +1167,18 @@ export class Datetime implements ComponentInterface {
      */
     raf(() => visibleIO?.observe(intersectionTrackerRef!));
 
+    /**
+     * TODO(FW-6931): Remove this fallback upon solving the root cause
+     * Fallback: If IntersectionObserver never reports that the
+     * datetime is visible but the host clearly has layout, ensure
+     * we still initialize listeners and mark the component as ready.
+     *
+     * We schedule this after everything has had a chance to run.
+     */
+    setTimeout(() => {
+      this.ensureReadyIfVisible();
+    }, 100);
+
     /**
      * We need to clean up listeners when the datetime is hidden
      * in a popover/modal so that we can properly scroll containers

core/src/components/datetime/test/basic/datetime.e2e.ts

@@ -394,6 +394,61 @@ configs({ modes: ['ios'], directions: ['ltr'] }).forEach(({ title, config }) =>
   });
 });
 
+/**
+ * Synthetic IntersectionObserver fallback behavior.
+ *
+ * This test stubs IntersectionObserver so that the callback
+ * never reports an intersecting entry. The datetime should
+ * still become ready via its internal fallback logic.
+ */
+configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, config }) => {
+  test.describe(title('datetime: IO fallback'), () => {
+    test('should become ready even if IntersectionObserver never reports visible', async ({ page }, testInfo) => {
+      testInfo.annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/30706',
+      });
+
+      await page.addInitScript(() => {
+        const OriginalIO = window.IntersectionObserver;
+        (window as any).IntersectionObserver = function (callback: any, options: any) {
+          const instance = new OriginalIO(() => {}, options);
+          const originalObserve = instance.observe.bind(instance);
+
+          instance.observe = (target: Element) => {
+            originalObserve(target);
+            callback([
+              {
+                isIntersecting: false,
+                target,
+              } as IntersectionObserverEntry,
+            ]);
+          };
+
+          return instance;
+        } as any;
+      });
+
+      await page.setContent(
+        `
+        <ion-datetime value="2022-05-03"></ion-datetime>
+      `,
+        config
+      );
+
+      const datetime = page.locator('ion-datetime');
+
+      // Give the fallback a short amount of time to run
+      await page.waitForTimeout(100);
+
+      await expect(datetime).toHaveClass(/datetime-ready/);
+
+      const calendarBody = datetime.locator('.calendar-body');
+      await expect(calendarBody).toBeVisible();
+    });
+  });
+});
+
 /**
  * We are setting RTL on the component
  * instead, so we don't need to test