fix(expansion): panel appearing as open when parent is animating away

Fixes an issue where the expansion panel will appear as if it is open, if the element is part of a component that is animating away. The issue comes from the fact that Angular resets the animation state to `void` which we don't have in the animation definitions.

Fixes #11765.

Author: crisbeto

src/lib/expansion/expansion-animations.ts

@@ -65,17 +65,19 @@ export const matExpansionAnimations: {
     }), {
       params: {expandedHeight: '64px'}
     }),
-    transition('expanded <=> collapsed, void => collapsed', group([
+    transition('expanded <=> collapsed, expanded <=> void, void => collapsed', group([
       query('@indicatorRotate', animateChild(), {optional: true}),
       animate(EXPANSION_PANEL_ANIMATION_TIMING),
     ])),
   ]),
 
   /** Animation that expands and collapses the panel content. */
   bodyExpansion: trigger('bodyExpansion', [
+    // Note: we also have `void` here as a fallback, because Angular will reset the
+    // state back to void when the element is being removed. See #11765.
     state('collapsed, void', style({height: '0px', visibility: 'hidden'})),
     state('expanded', style({height: '*', visibility: 'visible'})),
-    transition('expanded <=> collapsed, void => collapsed',
-      animate(EXPANSION_PANEL_ANIMATION_TIMING)),
+    transition('expanded <=> collapsed, expanded <=> void, void => collapsed',
+        animate(EXPANSION_PANEL_ANIMATION_TIMING)),
   ])
 };

src/lib/expansion/expansion-panel.ts

@@ -180,8 +180,7 @@ export class MatExpansionPanel extends CdkAccordionItem implements AfterContentI
     // with doing it via change detection.
     if (phaseName === 'done' && toState === 'expanded') {
       classList.add(cssClass);
-    }
-    if (phaseName === 'start' && toState === 'collapsed') {
+    } else if (phaseName === 'start' && (toState === 'collapsed' || toState === 'void')) {
       classList.remove(cssClass);
     }