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).

2 files changed, 8 insertions, 0 deletions

diff --git a/zellij-utils/src/input/config.rs b/zellij-utils/src/input/config.rs
index 9387aef3..546d39d0 100644
--- a/zellij-utils/src/input/config.rs
+++ b/zellij-utils/src/input/config.rs
@@ -1408,6 +1408,8 @@ mod config_test {
                 rounded_corners: true,
                 hide_session_name: true,
                 title_alignment: Default::default(),
+                gap: Default::default(),
+                padding: Default::default(),
             },
         };
         assert_eq!(config.ui, expected_ui_config, "Ui config defined in config");
diff --git a/zellij-utils/src/input/theme.rs b/zellij-utils/src/input/theme.rs
index 0930bc15..949a8752 100644
--- a/zellij-utils/src/input/theme.rs
+++ b/zellij-utils/src/input/theme.rs
@@ -27,6 +27,10 @@ pub struct FrameConfig {
     pub rounded_corners: bool,
     pub hide_session_name: bool,
     pub title_alignment: crate::data::PaneTitleAlignment,
+    /// outer gap between panes, in cells (see Style::pane_gap)
+    pub gap: usize,
+    /// inner padding between a pane's frame and its content (see Style::pane_padding)
+    pub padding: usize,
 }
 
 impl FrameConfig {
@@ -35,6 +39,8 @@ impl FrameConfig {
         merged.rounded_corners = other.rounded_corners;
         merged.hide_session_name = other.hide_session_name;
         merged.title_alignment = other.title_alignment;
+        merged.gap = other.gap;
+        merged.padding = other.padding;
         merged
     }
 }