sl0p: configurable pane gap (outer) and padding (inner)

Adds `ui { pane_frames { gap N; padding N } }`, both defaulting to 0 so
stock configs are unaffected.

  gap     - cells each pane gives up on every side, leaving real empty
            terminal between neighbouring panes (WM-style outer gaps).
            Two adjacent panes therefore show 2*gap between them.
  padding - cells between a pane's frame and its content; breathing room
            inside the pane without moving the frame.

Design: the pane KEEPS its allocated PaneGeom. Only two things move:

  1. the drawn frame, via Viewport::inset(gap), reusing the existing
     frame_geom_override mechanism already used for stacked panes;
  2. the content, via Offset::expanded_by(gap + padding) inside
     set_content_offset.

Because the allocation is untouched, the kasuari resize solver and mouse
hit-testing need no changes at all, and a click landing in a gap still
resolves to the owning pane. Content position, size and cursor are all
derived from content_offset, so widening it in one place keeps every
consumer consistent.

Viewport::inset refuses to shrink a pane below a frame's minimum viable
extent (two borders + one content cell) rather than emitting a degenerate
or zero-sized frame. Negative KDL values clamp to 0 instead of wrapping.

Tests cover: inset identity/shrink/recentre, the containment invariant
(an inset frame never escapes its allocation), small-pane refusal, and
the key safety property that content inset by gap+border+padding lands
strictly inside the frame drawn at inset(gap).

1 files changed, 33 insertions, 0 deletions

diff --git a/zellij-utils/src/kdl/mod.rs b/zellij-utils/src/kdl/mod.rs
index bc188aed..6589d30f 100644
--- a/zellij-utils/src/kdl/mod.rs
+++ b/zellij-utils/src/kdl/mod.rs
@@ -2482,6 +2482,17 @@ macro_rules! kdl_children_property_first_arg_as_bool {
 }
 
 #[macro_export]
+macro_rules! kdl_children_property_first_arg_as_i64 {
+    ( $kdl_node:expr, $property_name:expr ) => {
+        $kdl_node
+            .children()
+            .and_then(|c| c.get($property_name))
+            .and_then(|p| p.entries().iter().next())
+            .and_then(|p| p.value().as_i64())
+    };
+}
+
+#[macro_export]
 macro_rules! kdl_property_first_arg_as_i64 {
     ( $kdl_node:expr, $property_name:expr ) => {
         $kdl_node
@@ -5535,10 +5546,20 @@ impl UiConfig {
                 },
                 None => crate::data::PaneTitleAlignment::default(),
             };
+            // negative values are meaningless here and would wrap on `as usize`,
+            // so clamp at 0 rather than silently producing a gigantic inset
+            let gap = kdl_children_property_first_arg_as_i64!(pane_frames, "gap")
+                .unwrap_or(0)
+                .max(0) as usize;
+            let padding = kdl_children_property_first_arg_as_i64!(pane_frames, "padding")
+                .unwrap_or(0)
+                .max(0) as usize;
             let frame_config = FrameConfig {
                 rounded_corners,
                 hide_session_name,
                 title_alignment,
+                gap,
+                padding,
             };
             ui_config.pane_frames = frame_config;
         }
@@ -5575,6 +5596,18 @@ impl UiConfig {
             ));
             frame_config_children.nodes_mut().push(title_alignment);
         }
+        if self.pane_frames.gap > 0 {
+            has_ui_config = true;
+            let mut gap = KdlNode::new("gap");
+            gap.push(KdlValue::Base10(self.pane_frames.gap as i64));
+            frame_config_children.nodes_mut().push(gap);
+        }
+        if self.pane_frames.padding > 0 {
+            has_ui_config = true;
+            let mut padding = KdlNode::new("padding");
+            padding.push(KdlValue::Base10(self.pane_frames.padding as i64));
+            frame_config_children.nodes_mut().push(padding);
+        }
         if has_ui_config {
             frame_config.set_children(frame_config_children);
             ui_config_children.nodes_mut().push(frame_config);