fix(header): prevent flickering during iOS page transitions (#30705)

Issue number: resolves #25326

---------

<!-- 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. -->

The header flickers upon page transition when on iOS mode and using a
condensed header:

**Entering Page Two (P1 → P2):**
When navigating to Page Two, which has a collapsing header (intended to
be hidden until scroll), the header briefly flashes into view. This
happens because the header is initially rendered with full `opacity: 1`
before the component's logic can apply `opacity: 0` to hide it, causing
a visible flicker.

**Navigating Back (P2 → P1):**
When navigating back, Page One's header briefly bleeds through the top
of Page Two. Although Page Two is on top (`z−index: 100`), its
collapsing header is set to `opacity: 0`. This transparency allows Page
One header (`z−index: 99`) to become visible underneath, as the
transparent area cannot block the content below it.

The header flickers upon page transition when on iOS mode and using a
fade header:

**Entering Page Two (P1 → P2):**
When navigating to Page Two, which has a fade header (should not have a
background on load), the header background briefly flashes into view.
This happens because the header is initially rendered with full
`opacity: 1` before the component's logic can apply `opacity: 0` to hide
it, causing a visible flicker.

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

- Added a transition-specific class that is applied to the condensed
ion-header element to override its default transparency.

This guarantees the header to act as an opaque block during the page
transition, eliminating visual flickering caused by early `opacity: 0`
or the header underneath bleeding through.

- Added a transition-specific class that is applied to the fade
ion-header element to override its default opaque background.

This guarantees the header to act as a transparent block during the page
transition, eliminating visual flickering caused by default `opacity:
1`.


## 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/docs/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: `8.7.6-dev.11759524961.1cff6814`

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

@@ -39,6 +39,15 @@
   --opacity-scale: inherit;
 }
 
+/**
+ * Override styles applied during the page transition to prevent
+ * header flickering.
+ */
+.header-collapse-fade.header-transitioning ion-toolbar {
+  --background: transparent;
+  --border-style: none;
+}
+
 // iOS Header - Collapse Condense
 // --------------------------------------------------
 .header-collapse-condense {
@@ -65,8 +74,6 @@
  * since it needs to blend in with the header above it.
  */
 .header-collapse-condense ion-toolbar {
-  --background: var(--ion-background-color, #fff);
-
   z-index: 0;
 }
 
@@ -93,6 +100,28 @@
   transition: all 0.2s ease-in-out;
 }
 
+/**
+ * Large title toolbar should just use the content background
+ * since it needs to blend in with the header above it.
+ */
+.header-collapse-condense ion-toolbar,
+/**
+ * Override styles applied during the page transition to prevent
+ * header flickering.
+ */
+.header-collapse-condense-inactive.header-transitioning:not(.header-collapse-condense) ion-toolbar {
+  --background: var(--ion-background-color, #fff);
+}
+
+/**
+ * Override styles applied during the page transition to prevent
+ * header flickering.
+ */
+.header-collapse-condense-inactive.header-transitioning:not(.header-collapse-condense) ion-toolbar {
+  --border-style: none;
+  --opacity-scale: 1;
+}
+
 .header-collapse-condense-inactive:not(.header-collapse-condense) ion-toolbar.in-toolbar ion-title,
 .header-collapse-condense-inactive:not(.header-collapse-condense) ion-toolbar.in-toolbar ion-buttons.buttons-collapse {
   opacity: 0;

core/src/utils/transition/index.ts

@@ -18,34 +18,51 @@ const focusController = createFocusController();
 
 // TODO(FW-2832): types
 
+/**
+ * Executes the main page transition.
+ * It also manages the lifecycle of header visibility (if any)
+ * to prevent visual flickering in iOS. The flickering only
+ * occurs for a condensed header that is placed above the content.
+ *
+ * @param opts Options for the transition.
+ * @returns A promise that resolves when the transition is complete.
+ */
 export const transition = (opts: TransitionOptions): Promise<TransitionResult> => {
   return new Promise((resolve, reject) => {
     writeTask(() => {
-      beforeTransition(opts);
-      runTransition(opts).then(
-        (result) => {
-          if (result.animation) {
-            result.animation.destroy();
+      const transitioningInactiveHeader = getIosIonHeader(opts);
+      beforeTransition(opts, transitioningInactiveHeader);
+      runTransition(opts)
+        .then(
+          (result) => {
+            if (result.animation) {
+              result.animation.destroy();
+            }
+            afterTransition(opts);
+            resolve(result);
+          },
+          (error) => {
+            afterTransition(opts);
+            reject(error);
           }
-          afterTransition(opts);
-          resolve(result);
-        },
-        (error) => {
-          afterTransition(opts);
-          reject(error);
-        }
-      );
+        )
+        .finally(() => {
+          // Ensure that the header is restored to its original state.
+          setHeaderTransitionClass(transitioningInactiveHeader, false);
+        });
     });
   });
 };
 
-const beforeTransition = (opts: TransitionOptions) => {
+const beforeTransition = (opts: TransitionOptions, transitioningInactiveHeader: HTMLElement | null) => {
   const enteringEl = opts.enteringEl;
   const leavingEl = opts.leavingEl;
 
   focusController.saveViewFocus(leavingEl);
 
   setZIndex(enteringEl, leavingEl, opts.direction);
+  // Prevent flickering of the header by adding a class.
+  setHeaderTransitionClass(transitioningInactiveHeader, true);
 
   if (opts.showGoBack) {
     enteringEl.classList.add('can-go-back');
@@ -278,6 +295,40 @@ const setZIndex = (
   }
 };
 
+/**
+ * Add a class to ensure that the header (if any)
+ * does not flicker during the transition. By adding the
+ * transitioning class, we ensure that the header has
+ * the necessary styles to prevent the following flickers:
+ * 1. When entering a page with a condensed header, the
+ * header should never be visible. However,
+ * it briefly renders the background color while
+ * the transition is occurring.
+ * 2. When leaving a page with a condensed header, the
+ * header has an opacity of 0 and the pages
+ * have a z-index which causes the entering page to
+ * briefly show it's content underneath the leaving page.
+ * 3. When entering a page or leaving a page with a fade
+ * header, the header should not have a background color.
+ * However, it briefly shows the background color while
+ * the transition is occurring.
+ *
+ * @param header The header element to modify.
+ * @param isTransitioning Whether the transition is occurring.
+ */
+const setHeaderTransitionClass = (header: HTMLElement | null, isTransitioning: boolean) => {
+  if (!header) {
+    return;
+  }
+
+  const transitionClass = 'header-transitioning';
+  if (isTransitioning) {
+    header.classList.add(transitionClass);
+  } else {
+    header.classList.remove(transitionClass);
+  }
+};
+
 export const getIonPageElement = (element: HTMLElement) => {
   if (element.classList.contains('ion-page')) {
     return element;
@@ -291,6 +342,32 @@ export const getIonPageElement = (element: HTMLElement) => {
   return element;
 };
 
+/**
+ * Retrieves the ion-header element from a page based on the
+ * direction of the transition.
+ *
+ * @param opts Options for the transition.
+ * @returns The ion-header element or null if not found or not in 'ios' mode.
+ */
+const getIosIonHeader = (opts: TransitionOptions): HTMLElement | null => {
+  const enteringEl = opts.enteringEl;
+  const leavingEl = opts.leavingEl;
+  const direction = opts.direction;
+  const mode = opts.mode;
+
+  if (mode !== 'ios') {
+    return null;
+  }
+
+  const element = direction === 'back' ? leavingEl : enteringEl;
+
+  if (!element) {
+    return null;
+  }
+
+  return element.querySelector('ion-header');
+};
+
 export interface TransitionOptions extends NavOptions {
   progressCallback?: (ani: Animation | undefined) => void;
   baseEl: any;