fix(overlay): remove panelClass when the overlay is detached (#12142)

Removes the `panelClass` when the overlay is detached. This ensures that any overlay styling that targets the panel isn't left behind while the panel is detached.

Relates to #12099.

src/cdk/overlay/overlay-ref.ts

@@ -164,12 +164,16 @@ export class OverlayRef implements PortalOutlet, OverlayReference {
       this._config.scrollStrategy.disable();
     }
 
+    if (this._config.panelClass) {
+      this._toggleClasses(this._pane, this._config.panelClass, false);
+    }
+
     const detachmentResult = this._portalOutlet.detach();
 
     // Only emit after everything is detached.
     this._detachments.next();
 
-    // Remove this overlay from keyboard dispatcher tracking
+    // Remove this overlay from keyboard dispatcher tracking.
     this._keyboardDispatcher.remove(this);
 
     return detachmentResult;

src/cdk/overlay/overlay.spec.ts

@@ -628,6 +628,26 @@ describe('Overlay', () => {
       expect(pane.classList).toContain('custom-class-one');
       expect(pane.classList).toContain('custom-class-two');
     });
+
+    it('should remove the custom panel class when the overlay is detached', () => {
+      const config = new OverlayConfig({panelClass: 'custom-panel-class'});
+      const overlayRef = overlay.create(config);
+
+      overlayRef.attach(componentPortal);
+      viewContainerFixture.detectChanges();
+
+      const pane = overlayContainerElement.querySelector('.cdk-overlay-pane') as HTMLElement;
+      expect(pane.classList).toContain('custom-panel-class');
+
+      overlayRef.detach();
+      viewContainerFixture.detectChanges();
+      expect(pane.classList).not.toContain('custom-panel-class');
+
+      overlayRef.attach(componentPortal);
+      viewContainerFixture.detectChanges();
+      expect(pane.classList).toContain('custom-panel-class');
+    });
+
   });
 
   describe('scroll strategy', () => {