Increase feedback when editing mode is enabled

[simonprast]

Is your feature request related to a problem? Please describe.
You should be able to see that editing mode is enabled at the very first millisecond even after coming back from tripping on DMT during your lunch break.

Describe the solution you'd like
I think it would look cool if there's a thin border surrounding the browser window's edges, but keep in mind that this border should not take up physical space of the layout (just an overlay).