Add option for more subtle pane focus border

[Studio384]

Description of the new feature/enhancement

I feel like the images in #3060 are quit misleading. While the accent border isn't distracting in these images because of the bright colors set for the terminal, using normal terminal settings, the accent border is screaming for attention.

Proposed technical implementation details (optional)

I like the idea of using the accent color, but putting a border all the way around the terminal is a no-go. Perhaps a look should be taken to Windows 8's active app indication?

This should be an obvious indication, but it should not be distracting.

[zadjii-msft]

I'm not totally sure about what's being reported here -
Are you referring to the "active pane border" which separates two panes, and identifies which one is focused?

Enabling users to configure this color is being tracked in #3061

Or the "window border"

[Studio384]

I am referring to the pane borders, as per the description.

[zadjii-msft]

@Studio384 Just making sure - I think the "putting a border all the way around the terminal" threw me for a loop, especially since the window border bot added in 0.7.

Would you think that being able to set the color of the pane border manually would be enough to mitigate this? If so, then #3061 is the issue for you ☺️

[Studio384]

I'd actually argue for a different indication style, perhaps something like what Windows 8.1 used to do when using Snap, A more subtle line instead of the entire border. There is nothing wrong with the color, I'm even all for it, its just the size of the indication.

[zadjii-msft]

I don't hate that idea. Might be a bit trickier to pull off, but I don't see why not.

For future self:

That little three dots with the line is the way it looked in 8.1.