Add segmentation for separator dividers

[max-legrand]

Add segmentation to better highlight only the active pane

Screenshots

[rockorager]

Thanks for working on this! The feature is useful - tmux-style segmented borders that highlight the active pane path are a nice UX improvement.

However, after reviewing the implementation, I think it's heavier than it needs to be for what it accomplishes. Here are some concerns and suggestions:

1. Unrelated behavior change in focus navigation

The PR changes move_focus to use get_preferred_leaf() instead of directional get_first_leaf()/get_last_leaf():

-- Before:
if forward then
    target_leaf = get_first_leaf(sibling_node)
else
    target_leaf = get_last_leaf(sibling_node)
end

-- After:
local target_leaf = get_preferred_leaf(sibling_node)

This means left/right/up/down focus moves no longer respect direction - they always go to the last-focused leaf in that subtree. This is a real behavior change unrelated to border rendering and could surprise users.

Suggestion: Either keep the old behavior for navigation, or split this into a separate PR with clear justification.

2. New widget type may be unnecessary

Adding SegmentedSeparator as a new widget kind requires changes in many places (destroy, layout, paint, hit regions, etc). Consider extending the existing Separator instead:

pub const Separator = struct {
    axis: Axis,
    style: vaxis.Style = .{},
    border: Box.Border = .single,
    segments: ?[]const SeparatorSegment = null, // optional
};

In Lua:

prise.Separator({
    axis = sep_axis,
    border = config.borders.style,
    segments = segments,  -- optional, when needed
    style = { fg = default_color },
})

This keeps the concept simple: "it's just a separator, sometimes with segments."

3. Over-engineered segment API

The current API supports both ratios AND absolute positions:

---@field ratio_start? number Start position as ratio (0.0-1.0)
---@field ratio_end? number End position as ratio (0.0-1.0)
---@field start? number Absolute start position
---@field end? number Absolute end position

But only ratios are actually used. This adds complexity (the is_ratio bool, 1000x scaling, etc). Suggest dropping absolute support until actually needed.

4. Asymmetric handling of splits

create_divider_segments has:

if left_is_col_split then
    ...
elseif right_is_col_split then
    ...
end

If both sides are splits, only the left side is considered. Suggest explicitly handling "both sides split" by falling back to simple separator coloring:

if left_is_col_split and right_is_col_split then
    return {}
end

5. Missing tests

No tests for:

• create_divider_segments behavior for various layouts
• Serialization of last_focused_child_idx
• Edge cases in segmentation logic

---

Overall I think this could be simplified to ~200-300 lines by:

1. Extending Separator instead of new widget type
2. Only supporting ratio-based segments
3. Decoupling focus navigation changes
4. Using simpler contains_focused heuristic for v1

Happy to discuss further!

[rockorager]

(Note that this was an AI generated review...need to fix that this posts as me)

[rockorager]

See previous review comment