1 files changed, 15 insertions, 3 deletions
diff --git a/src/layout.zig b/src/layout.zig
index 8db05a07..3637f30e 100644
--- a/src/layout.zig
+++ b/src/layout.zig
@@ -713,14 +713,26 @@ pub fn columnBySerial(p: *const Pardes, serial: u32) ?usize {
     return null;
 }
 
-pub fn columnMoveTarget(p: *const Pardes, source: usize, x: u16) usize {
-    if (source >= p.ncol) return source;
+/// What releasing column `source`'s grip at `x` does: past a neighbour's
+/// middle it takes that column's place; short of it, as in acme, the
+/// column's left edge moves to `x - grab`, clamped as setColumnPairWidth
+/// clamps it. Still on the grip, or at the leftmost column, nothing moves.
+/// The drag preview and the release both ask here, so they cannot disagree.
+pub const ColumnDrop = union(enum) { stay, reorder: usize, edge: u16 };
+
+pub fn columnDrop(p: *const Pardes, source: usize, x: u16, grab: u16) ColumnDrop {
+    if (source >= p.ncol) return .stay;
     var insertion: usize = 0;
     for (0..p.ncol) |column| {
         if (column == source) continue;
         if (x >= p.col_x[column] + p.col_w[column] / 2) insertion += 1;
     }
-    return insertion;
+    if (insertion != source) return .{ .reorder = insertion };
+    if (source == 0 or (x >= p.col_x[source] and x < p.col_x[source] + config.GUTTER)) return .stay;
+    if (p.col_w[source - 1] + p.col_w[source] < 2 * config.MINW) return .stay;
+    const lo = p.col_x[source - 1] + config.MINW;
+    const hi = (p.col_x[source] + p.col_w[source]) -| config.MINW;
+    return .{ .edge = std.math.clamp(x -| grab, lo, @max(lo, hi)) };
 }
 
 /// Share out the width of columns `left` and `left + 1` so the left one is