Restore focus on description page when it regains focus

[mjbvz]

From microsoft/vscode#48604

• Extension version: 1.74.0
• VSCode Version: 1.33 - 73ccd28be6b94b8965b65e6f187689dca7c30805
• OS: Mac OS 10.14.3

Steps to Reproduce:

1. Open the description page for a PR
2. Start typing in the comment box
3. Switch focus to another editor group in VS Code (keeping the description page visible)
4. Switch focus back to the description page

Bug
Focus is not restored to the comment box.