fix(header): avoid flicker on collapsible header load (#19682)

* fix flicker on collapse load

* remove old test

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

@@ -51,8 +51,8 @@
   padding-bottom: 13px;
 }
 
-ion-toolbar.in-toolbar ion-title,
+.header-collapse-main ion-toolbar.in-toolbar ion-title,
-ion-toolbar.in-toolbar ion-buttons {
+.header-collapse-main ion-toolbar.in-toolbar ion-buttons {
   transition: all 0.2s ease-in-out;
 }
 

core/src/components/header/header.tsx

@@ -2,7 +2,7 @@ import { Component, ComponentInterface, Element, Host, Prop, h, readTask, writeT
 
 import { getIonMode } from '../../global/ionic-global';
 
-import { cloneElement, createHeaderIndex, handleContentScroll, handleToolbarIntersection, setHeaderActive } from './header.utils';
+import { cloneElement, createHeaderIndex, handleContentScroll, handleToolbarIntersection, setHeaderActive, setToolbarBackgroundOpacity } from './header.utils';
 /**
  * @virtualProp {"ios" | "md"} mode - The mode determines which platform styles to use.
  */
@@ -19,6 +19,7 @@ export class Header implements ComponentInterface {
   private scrollEl?: HTMLElement;
   private contentScrollCallback?: any;
   private intersectionObserver?: any;
+  private collapsibleMainHeader?: HTMLElement;
 
   @Element() el!: HTMLElement;
 
@@ -77,6 +78,11 @@ export class Header implements ComponentInterface {
       this.scrollEl.removeEventListener('scroll', this.contentScrollCallback);
       this.contentScrollCallback = undefined;
     }
+
+    if (this.collapsibleMainHeader) {
+      this.collapsibleMainHeader.classList.remove('header-collapse-main');
+      this.collapsibleMainHeader = undefined;
+    }
   }
 
   private async setupCollapsibleHeader(contentEl: HTMLIonContentElement | null, pageEl: Element | null) {
@@ -84,18 +90,18 @@ export class Header implements ComponentInterface {
 
     this.scrollEl = await contentEl.getScrollElement();
 
-    readTask(() => {
     const headers = pageEl.querySelectorAll('ion-header');
-      const mainHeader = Array.from(headers).find((header: any) => header.collapse !== 'condense') as HTMLElement | undefined;
+    this.collapsibleMainHeader = Array.from(headers).find((header: any) => header.collapse !== 'condense') as HTMLElement | undefined;
 
-      if (!mainHeader || !this.scrollEl) { return; }
+    if (!this.collapsibleMainHeader) { return; }
 
-      const mainHeaderIndex = createHeaderIndex(mainHeader);
+    const mainHeaderIndex = createHeaderIndex(this.collapsibleMainHeader);
     const scrollHeaderIndex = createHeaderIndex(this.el);
 
     if (!mainHeaderIndex || !scrollHeaderIndex) { return; }
 
     setHeaderActive(mainHeaderIndex, false);
+    setToolbarBackgroundOpacity(mainHeaderIndex.toolbars[0], 0);
 
     readTask(() => {
         const mainHeaderHeight = mainHeaderIndex.el.clientHeight;
@@ -109,7 +115,6 @@ export class Header implements ComponentInterface {
         const toolbarIntersection = (ev: any) => { handleToolbarIntersection(ev, mainHeaderIndex, scrollHeaderIndex); };
         this.intersectionObserver = new IntersectionObserver(toolbarIntersection, { threshold: [0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1], rootMargin: `-${mainHeaderHeight}px 0px 0px 0px` });
         this.intersectionObserver.observe(scrollHeaderIndex.toolbars[0].el);
-      });
 
       /**
        * Handle scaling of large iOS titles and
@@ -117,12 +122,14 @@ export class Header implements ComponentInterface {
        * in primary header
        */
         this.contentScrollCallback = () => { handleContentScroll(this.scrollEl!, scrollHeaderIndex); };
-      this.scrollEl.addEventListener('scroll', this.contentScrollCallback);
+        this.scrollEl!.addEventListener('scroll', this.contentScrollCallback);
     });
 
     writeTask(() => {
       cloneElement('ion-title');
       cloneElement('ion-back-button');
+
+      this.collapsibleMainHeader!.classList.add('header-collapse-main');
     });
 
     this.collapsibleHeaderInitialized = true;

core/src/components/header/header.utils.ts

@@ -45,7 +45,7 @@ export const createHeaderIndex = (headerEl: HTMLElement | undefined): HeaderInde
         innerTitleEl: (ionTitleEl) ? ionTitleEl.shadowRoot!.querySelector('.toolbar-title') : null,
         ionButtonsEl: Array.from(toolbar.querySelectorAll('ion-buttons')) || []
       } as ToolbarIndex;
-    }) || [[]]
+    }) || []
   } as HeaderIndex;
 };
 
@@ -60,7 +60,7 @@ export const handleContentScroll = (scrollEl: HTMLElement, scrollHeaderIndex: He
   });
 };
 
-const setToolbarBackgroundOpacity = (toolbar: ToolbarIndex, opacity: number | undefined) => {
+export const setToolbarBackgroundOpacity = (toolbar: ToolbarIndex, opacity: number | undefined) => {
   if (opacity === undefined) {
     toolbar.background.style.removeProperty('--opacity');
   } else {
@@ -124,13 +124,11 @@ export const handleToolbarIntersection = (ev: any, mainHeaderIndex: HeaderIndex,
 };
 
 export const setHeaderActive = (headerIndex: HeaderIndex, active = true) => {
-  writeTask(() => {
   if (active) {
     headerIndex.el.classList.remove('header-collapse-condense-inactive');
   } else {
     headerIndex.el.classList.add('header-collapse-condense-inactive');
   }
-  });
 };
 
 export const scaleLargeTitles = (toolbars: ToolbarIndex[] = [], scale = 1, transition = false) => {