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, 4 insertions, 0 deletions

diff --git a/zellij-utils/src/plugin_api/event.rs b/zellij-utils/src/plugin_api/event.rs
index 0189d11b..f4bb3523 100644
--- a/zellij-utils/src/plugin_api/event.rs
+++ b/zellij-utils/src/plugin_api/event.rs
@@ -2493,6 +2493,8 @@ fn serialize_mode_update_event_with_non_default_values() {
             rounded_corners: true,
             hide_session_name: false,
             pane_title_alignment: Default::default(),
+            pane_gap: Default::default(),
+            pane_padding: Default::default(),
         },
         capabilities: PluginCapabilities { arrow_fonts: false },
         session_name: Some("my awesome test session".to_owned()),
diff --git a/zellij-utils/src/plugin_api/style.rs b/zellij-utils/src/plugin_api/style.rs
index 2f3ff298..46c3ce83 100644
--- a/zellij-utils/src/plugin_api/style.rs
+++ b/zellij-utils/src/plugin_api/style.rs
@@ -26,6 +26,8 @@ impl TryFrom<ProtobufStyle> for Style {
             // alignment, and adding a proto field would mean regenerating the
             // committed prost assets.
             pane_title_alignment: Default::default(),
+            pane_gap: Default::default(),
+            pane_padding: Default::default(),
         })
     }
 }