fix(material/expansion): Expansion panel content flickers during animation for lazy-loaded custom height nested expansion panels

This fix introduces workaround for a `void => collapsed` animation that is causing improper behaviour for lazy-loaded body.
First, it disables first `void => collapsed` animation after init to disable recalculation which fixes changing heights of inner panels.
Second, it sets `mat-expansion-panel-hidden` class on content init and removes it on creating lazy-loaded body to remove flicker of parent expansion panel

Fixes #22715

Author: xAmaris

src/material/expansion/expansion-panel.html

@@ -1,11 +1,15 @@
 <ng-content select="mat-expansion-panel-header"></ng-content>
-<div class="mat-expansion-panel-content"
-     role="region"
-     [@bodyExpansion]="_getExpandedState()"
-     (@bodyExpansion.done)="_bodyAnimationDone.next($event)"
-     [attr.aria-labelledby]="_headerId"
-     [id]="id"
-     #body>
+<div
+  class="mat-expansion-panel-content"
+  role="region"
+  [class.mat-expansion-panel-hidden]="_panelHidden"
+  [@bodyExpansion]="_getExpandedState()"
+  (@bodyExpansion.start)="_animationStarted()"
+  (@bodyExpansion.done)="_bodyAnimationDone.next($event)"
+  [attr.aria-labelledby]="_headerId"
+  [id]="id"
+  #body
+>
   <div class="mat-expansion-panel-body">
     <ng-content></ng-content>
     <ng-template [cdkPortalOutlet]="_portal"></ng-template>

src/material/expansion/expansion-panel.scss

@@ -62,6 +62,10 @@
   }
 }
 
+.mat-expansion-panel-hidden {
+  height: 0;
+}
+
 .mat-expansion-panel-body {
   padding: 0 24px 16px;
 }

src/material/expansion/expansion-panel.ts

@@ -95,6 +95,7 @@ export const MAT_EXPANSION_PANEL_DEFAULT_OPTIONS =
     '[class.mat-expanded]': 'expanded',
     '[class._mat-animation-noopable]': '_animationMode === "NoopAnimations"',
     '[class.mat-expansion-panel-spacing]': '_hasSpacing()',
+    '[@.disabled]': '_animationsDisabled',
   },
 })
 export class MatExpansionPanel
@@ -150,6 +151,12 @@ export class MatExpansionPanel
   /** Stream of body animation done events. */
   readonly _bodyAnimationDone = new Subject<AnimationEvent>();
 
+  /** Whether Angular animations in the panel body should be disabled. */
+  _animationsDisabled = false;
+
+  /** Whether panel body should be hidden. */
+  _panelHidden = false;
+
   constructor(
     @Optional() @SkipSelf() @Inject(MAT_ACCORDION) accordion: MatAccordionBase,
     _changeDetectorRef: ChangeDetectorRef,
@@ -218,6 +225,9 @@ export class MatExpansionPanel
 
   ngAfterContentInit() {
     if (this._lazyContent && this._lazyContent._expansionPanel === this) {
+      this._panelHidden = !this.expanded;
+      this._animationsDisabled = !this.expanded;
+
       // Render the content as soon as the panel becomes open.
       this.opened
         .pipe(
@@ -227,6 +237,7 @@ export class MatExpansionPanel
         )
         .subscribe(() => {
           this._portal = new TemplatePortal(this._lazyContent._template, this._viewContainerRef);
+          this._hidePanel();
         });
     }
   }
@@ -251,6 +262,31 @@ export class MatExpansionPanel
 
     return false;
   }
+
+  _animationStarted() {
+    // Currently the `bodyExpansion` animation has a `void => collapsed` transition which is
+    // there to work around a bug in Angular (see #13088), however this introduces a different
+    // issue. The new transition will cause to flicker in certain situations (see #22715), if the
+    // consumer has set a inner lazy-loaded expansion panel's header height that is different from the
+    // default one.
+    // Part of work around is to disable animations on the body and re-enabling them after the first animation has run.
+    // Ideally this wouldn't be necessary if we remove the `void => collapsed` transition, but we have
+    // to wait for https://github.com/angular/angular/issues/18847 to be resolved.
+    this._animationsDisabled = false;
+  }
+
+  private _hidePanel(): void {
+    // Currently the `bodyExpansion` animation has a `void => collapsed` transition which is
+    // there to work around a bug in Angular (see #13088), however this introduces a different
+    // issue. The new transition will cause to flicker in certain situations (see #22715), if the
+    // consumer has set a inner lazy-loaded expansion panel's header height that is different from the
+    // default one.
+    // Part of work around is to set non-expanded panel's height to 0px with class on init
+    // so that outer expansion panel can calculate height of its body properly.
+    // Ideally this wouldn't be necessary if we remove the `void => collapsed` transition, but we have
+    // to wait for https://github.com/angular/angular/issues/18847 to be resolved.
+    this._panelHidden = false;
+  }
 }
 
 /**