Add idle split line to PaneGrid style #1186
Description
Currently pane_grid::Style only has hovered_split and picked_split — there's no way to render a visible divider between panes at rest. The spacing gap between panes is completely invisible, which makes it hard to tell where one pane ends and the next begins (or that a split even exists).
It would be nice to have an optional idle/resting split line style so apps using PaneGrid can show a subtle divider without needing to hack around it at the application level.
This came up in cosmic-term (pop-os/cosmic-term#607) but it affects any COSMIC app that uses PaneGrid with splits.
I took a stab at it in my fork if it's helpful as a starting point: Lcstyle/iced@feat/pane-grid-split-line — adds an Option<Line> field to Style and draws it in draw() for all split regions.
Screenshot
With the split line enabled (1px divider using the theme's bg_divider() color):
