Pressing Ctrl+A/Cmd+A in DevTools "selects" the whole iframe and is difficult to remove #4363

From flutter/devtools#5107.

There appear to a few causes:

1. The outer page has focus by default
2. The outer page has focus if you focus DevTools then tab away, then click back into the VS Code window
3. Pressing Ctrl+A inside DevTools is passed up to the parent page (used to ensure you can access the palette etc.)