fix(block-scroll-strategy): disable smooth scrolling before restoring scroll position

Disables user-defined `scroll-behavior: smooth` before restoring the scroll position when disabling the `BlockScrollStrategy`, which prevents browsers from animating the scroll position change.

Fixes #7139.

Author: crisbeto

src/cdk/overlay/scroll/block-scroll-strategy.spec.ts

@@ -136,6 +136,24 @@ describe('BlockScrollStrategy', () => {
     expect(document.documentElement.getBoundingClientRect().width).toBe(previousContentWidth);
   });
 
+  it('should not clobber user-defined scroll-behavior', skipIOS(() => {
+    const root = document.documentElement;
+    const body = document.body;
+
+    root.style['scrollBehavior'] = body.style['scrollBehavior'] = 'smooth';
+
+    // Get the value via the style declaration in order to
+    // handle browsers that don't support the property yet.
+    const initialRootValue = root.style['scrollBehavior'];
+    const initialBodyValue = root.style['scrollBehavior'];
+
+    overlayRef.attach(componentPortal);
+    overlayRef.detach();
+
+    expect(root.style['scrollBehavior']).toBe(initialRootValue);
+    expect(body.style['scrollBehavior']).toBe(initialBodyValue);
+  }));
+
   /**
    * Skips the specified test, if it is being executed on iOS. This is necessary, because
    * programmatic scrolling inside the Karma iframe doesn't work on iOS, which renders these

src/cdk/overlay/scroll/block-scroll-strategy.ts

@@ -45,11 +45,25 @@ export class BlockScrollStrategy implements ScrollStrategy {
   /** Unblocks page-level scroll while the attached overlay is open. */
   disable() {
     if (this._isEnabled) {
+      const html = document.documentElement;
+      const body = document.body;
+      const previousHTMLScrollBehavior = html.style['scrollBehavior'] || '';
+      const previousBodyScrollBehavior = body.style['scrollBehavior'] || '';
+
       this._isEnabled = false;
-      document.documentElement.style.left = this._previousHTMLStyles.left;
-      document.documentElement.style.top = this._previousHTMLStyles.top;
-      document.documentElement.classList.remove('cdk-global-scrollblock');
+
+      html.style.left = this._previousHTMLStyles.left;
+      html.style.top = this._previousHTMLStyles.top;
+      html.classList.remove('cdk-global-scrollblock');
+
+      // Disable user-defined smooth scrolling temporarily while we restore the scroll position.
+      // See https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-behavior
+      html.style['scrollBehavior'] = body.style['scrollBehavior'] = 'auto';
+
       window.scroll(this._previousScrollPosition.left, this._previousScrollPosition.top);
+
+      html.style['scrollBehavior'] = previousHTMLScrollBehavior;
+      body.style['scrollBehavior'] = previousBodyScrollBehavior;
     }
   }