fix(header): iOS headers in MD app are not hidden (#29164)

Issue number: resolves #28867

---------

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

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

I missed an edge case in
https://github.com/ionic-team/ionic-framework/pull/28277 that caused an
MD headers in an MD app to be hidden due to the presence of an iOS
header (via `mode="ios"`). This was happening because I forgot to scope
the selector in `header.ios.scss` to only iOS headers.

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

- The headers in the relevant selector are now scoped to the iOS headers
which avoids this bug while preserving the anti-flicker mechanism added
in the linked PR.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!--
  If this introduces a breaking change:
1. Describe the impact and migration path for existing applications
below.
  2. Update the BREAKING.md file with the breaking change.
3. Add "BREAKING CHANGE: [...]" to the commit description when merging.
See
https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#footer
for more information.
-->


## 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: `7.8.1-dev.11710452743.1ca99e5e`

---------

Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/header/header.ios.scss

@@ -132,8 +132,11 @@
  * We use opacity: 0 to avoid a layout shift.
  * We target both the attribute and the class in the event that the attribute
  * is not reflected on the host in some frameworks.
+ *
+ * Both headers should be scoped to iOS mode otherwise an MD app that uses an
+ * iOS header may cause other MD headers to be unexpectedly hidden.
  */
-ion-header:not(.header-collapse-main):has(~ ion-content ion-header[collapse="condense"], 
-~ ion-content ion-header.header-collapse-condense) {
+ion-header.header-ios:not(.header-collapse-main):has(~ ion-content ion-header.header-ios[collapse="condense"],
+~ ion-content ion-header.header-ios.header-collapse-condense) {
   opacity: 0;
 }

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

@@ -85,3 +85,91 @@ configs({ modes: ['ios'], directions: ['ltr'] }).forEach(({ title, screenshot, c
     });
   });
 });
+
+/**
+ * This test only impacts MD applications
+ */
+configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
+  test.describe(title('header: translucent'), () => {
+    test('should not hide MD headers when using a descendant iOS header in an MD app', async ({ page }) => {
+      test.info().annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/28867',
+      });
+      await page.setContent(
+        `
+        <ion-header id="main-header">
+          <ion-toolbar>
+            <ion-title>Header</ion-title>
+          </ion-toolbar>
+        </ion-header>
+        <ion-content>
+          <ion-header collapse="condense">
+            <ion-toolbar>
+              <ion-title size="large">Header</ion-title>
+            </ion-toolbar>
+          </ion-header>
+
+          <ion-header mode="ios">
+            <ion-toolbar>
+              <ion-title>Welcome</ion-title>
+            </ion-toolbar>
+          </ion-header>
+        </ion-content>
+      `,
+        config
+      );
+
+      const header = page.locator('ion-header#main-header');
+
+      /**
+       * The existence of the iOS header in an MD app should not cause the main MD header
+       * to be hidden. We do not have toHaveVisible because the behavior that hides
+       * the header under correct circumstances does it using opacity: 0.
+       * Playwright considers an element with opacity: 0 to still be visible
+       * because it has a non-zero bounding box.
+       */
+      await expect(header).toHaveScreenshot(screenshot('header-md-visibility-ios-descendant'));
+    });
+    test('should not hide MD headers when using a root iOS header in an MD app', async ({ page }) => {
+      test.info().annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/28867',
+      });
+      await page.setContent(
+        `
+        <ion-header id="main-header" mode="ios">
+          <ion-toolbar>
+            <ion-title>Header</ion-title>
+          </ion-toolbar>
+        </ion-header>
+        <ion-content>
+          <ion-header collapse="condense">
+            <ion-toolbar>
+              <ion-title size="large">Header</ion-title>
+            </ion-toolbar>
+          </ion-header>
+
+          <ion-header>
+            <ion-toolbar>
+              <ion-title>Welcome</ion-title>
+            </ion-toolbar>
+          </ion-header>
+        </ion-content>
+      `,
+        config
+      );
+
+      const header = page.locator('ion-header#main-header');
+
+      /**
+       * The existence of the iOS header in an MD app should not cause the main MD header
+       * to be hidden. We do not have toHaveVisible because the behavior that hides
+       * the header under correct circumstances does it using opacity: 0.
+       * Playwright considers an element with opacity: 0 to still be visible
+       * because it has a non-zero bounding box.
+       */
+      await expect(header).toHaveScreenshot(screenshot('header-md-visibility-ios-main'));
+    });
+  });
+});