[Feature] Add a mark for notebook spilt pane to indicate I'm using that pane

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Describe the solution you'd like?

Currently if I'm using split panel for terminals, there's a tiny triangle mark to indicate which pane I'm using:

So I can always know where I am when there are multiple panes:

However if this is a notebook pane there's no such mark:

The cursor is now on the notebook pane but the triangle mark is still on the terminal pane.

This makes it real hard to tell where I'm at and I sometimes close the wrong pane because of that, which is pretty annoying. Sure you can tell which pane you are using by looking for the cursor, but that's much harder than just check the triangle sign on the left upper corner.

Is your feature request related to a problem? Please describe.

No response

Additional context

No response

How important is this feature to you?

4

Warp Internal (ignore) - linear-label:39cc6478-1249-4ee7-950b-c428edfeecd1

None

No files, tests, or entry points are named. Start by locating the split-pane UI handling terminal and notebook panes, then compare how the active-pane triangle is selected and rendered. Done means the indicator follows the notebook pane when it is active without breaking terminal-pane behavior.