From f6d2af17fc922483802d6d91f4b73954628f6e1c Mon Sep 17 00:00:00 2001
From: George Kalpakas <kalpakas.g@gmail.com>
Date: Wed, 29 Jul 2020 17:38:11 +0300
Subject: [PATCH] fix(docs-infra): preserve focus on copy (and prevent
 scrolling to bottom on IE11) (#38244)

The `CopierService` is used for copying text to the user's clipboard. It
is, for example, used in `CodeComponent` to copy example code snippets.
This is implemented by creating a temporary, hidden `<textarea>`
elements, setting its value to the text that needs to be copied,
executing the `copy` command and finally removing the element from the
DOM.

Previously, as a result of `CopierService`'s implementation, the focused
element would lose focus, while the temporary `<textarea>` element would
implicitly gain focus when selecting its contents. This had an even
worse side-effect on IE11, which seems to scroll to the bottom of the
containing element (here `<body>`) when the focused element is removed.

This commit fixes these issues by keeping track of the previously
focused element and restoring its focus after the copy operation.

NOTE: This fix is inspired by Angular CDK's [PendingCopy][1] class.

[1]: https://github.com/angular/components/blob/89b5fa89d1437c3054c5/src/cdk/clipboard/pending-copy.ts

Fixes #37796

PR Close #38244
---
 aio/src/app/shared/copier.service.ts | 9 +++++++++
 1 file changed, 9 insertions(+)
diff --git a/aio/src/app/shared/copier.service.ts b/aio/src/app/shared/copier.service.ts
index 6b62ca7b3b..420d8ef683 100644
--- a/aio/src/app/shared/copier.service.ts
+++ b/aio/src/app/shared/copier.service.ts
@@ -5,6 +5,9 @@
  * - https://github.com/zenorocha/clipboard.js/
  *
  * Both released under MIT license - © Zeno Rocha
+ *
+ * It is also influenced by the Angular CDK `PendingCopy` class:
+ * https://github.com/angular/components/blob/master/src/cdk/clipboard/pending-copy.ts
  */
 
 
@@ -18,6 +21,8 @@ export class CopierService {
    * @return Whether the copy operation was successful.
    */
   private copyTextArea(textArea: HTMLTextAreaElement): boolean {
+    const currentFocus = document.activeElement as HTMLOrSVGElement | null;
+
     try {
       textArea.select();
       textArea.setSelectionRange(0, textArea.value.length);
@@ -25,6 +30,10 @@ export class CopierService {
       return document.execCommand('copy');
     } catch {
       return false;
+    } finally {
+      // Calling `.select()` on the `<textarea>` element may have also focused it.
+      // Change the focus back to the previously focused element.
+      currentFocus?.focus();
     }
   }
 
