[Playground] Unable to scroll code editor in selecting the text

How to reproduce:

1. Open Playground page with code that takes more than 1 page.
2. Put carriage at the end of visible code
3. Press the SHIFT button on the keyboard
4. Click the DOWN button on the keyboard

The editor isn't scrolled down. The same behavioral using mouse selecting.

Imported from Jira [BEAM-13047](https://issues.apache.org/jira/browse/BEAM-13047). Original Jira may contain additional context.
Reported by: IKozyrev.