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.tsFixes#37796
PR Close#38244
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