sl0p: optionally render the stacked-pane picker as a grid

`stack_list_layout "grid"` packs picker entries several per row, which turns
stack_list_max_rows from a count into a space budget: eight panes in a wide
terminal now fit on one row instead of spending three and hiding five.

The entry renderer already drew each entry as a single chunk at its own x/y,
sized by its geom and centring its own label, so a grid needed no rendering
changes at all -- only the loop assigning every entry the same x and the full
width. Column count is derived from the widest label rather than configured, so
it adapts to the titles; one column is the floor and reproduces the previous
full-width rows byte for byte.

The row -> member mapping now has a column axis, and it had already caused one
wrong-pane bug when two consumers disagreed. So the layout is computed once,
by one function, into explicit cells; the renderer, the reserved-row budget and
the mouse hit-test all read those same cells rather than each doing their own
arithmetic. Windowing is by row, so a grid scrolls a whole row at a time and
degrades to the old per-member window when there is one column.

Cells are separated by a two-column gap: without it the focused entry's "> "
marker sits flush against the previous cell and reads as belonging to it.

2 files changed, 6 insertions, 0 deletions

diff --git a/zellij-utils/src/input/config.rs b/zellij-utils/src/input/config.rs
index d64e4598..4cb315ca 100644
--- a/zellij-utils/src/input/config.rs
+++ b/zellij-utils/src/input/config.rs
@@ -1497,6 +1497,7 @@ mod config_test {
                 responsive_min_cols: 0,
                 responsive_layout: String::from("stacked"),
                 stack_list_max_rows: 0,
+                stack_list_grid: false,
             },
         };
         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 7cd89d41..412aaae2 100644
--- a/zellij-utils/src/input/theme.rs
+++ b/zellij-utils/src/input/theme.rs
@@ -52,6 +52,9 @@ pub struct FrameConfig {
     /// windows around the focused entry and reports the rest as "+N". 0 = show
     /// every member, however many rows that takes.
     pub stack_list_max_rows: usize,
+    /// Pack picker entries into a grid instead of one per row. The row cap then
+    /// becomes a space budget rather than a count.
+    pub stack_list_grid: bool,
 }
 
 /// Hand-written rather than derived: `gap_aspect` must default to 2 to match
@@ -72,6 +75,7 @@ impl Default for FrameConfig {
             responsive_min_cols: 0,
             responsive_layout: String::from("stacked"),
             stack_list_max_rows: 0,
+            stack_list_grid: false,
         }
     }
 }
@@ -103,6 +107,7 @@ impl FrameConfig {
         merged.responsive_min_cols = other.responsive_min_cols;
         merged.responsive_layout = other.responsive_layout.clone();
         merged.stack_list_max_rows = other.stack_list_max_rows;
+        merged.stack_list_grid = other.stack_list_grid;
         merged
     }
 }