Grid density: a coarser board settles its tiles instead of stacking them

Stepping the grid coarser pulled tiles past the right edge back in and
left them wherever that put them — on top of one another, which the
board never allows under a drag. Tiles now settle in reading order: one
that would land on a tile already settled takes the first free spot, the
same rule a newly added tile follows.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/shell.js

@@ -296,8 +296,9 @@ function cellHeightFor(cols) {
 }
 
 /** Cut the board into `cols` columns. Tiles keep their grid units — the
-    board gets denser or coarser and everything on it changes size with it;
-    only a tile now past the right edge is pulled back in. */
+    board gets denser or coarser and everything on it changes size with it.
+    A tile now past the right edge is pulled back in, and one that would then
+    sit on another moves to the first free spot instead. */
 function setGridCols(value, { save = true } = {}) {
   const next = normalizeGridCols(value);
   const changed = next !== COLS;
@@ -305,9 +306,16 @@ function setGridCols(value, { save = true } = {}) {
   grid.style.gridTemplateColumns = `repeat(${COLS}, minmax(0, 1fr))`;
   document.documentElement.style.setProperty('--cell-h', cellHeightFor(COLS) + 'px');
   if (changed) {
-    for (const tile of tiles) {
+    // A coarser board may not have room for every tile where it was. Settle
+    // them in reading order: pull a tile past the right edge back in, and
+    // give one that would then land on a tile already settled the first free
+    // spot instead — tiles never overlap here, any more than under a drag.
+    const settled = [];
+    for (const tile of tiles.slice().sort((a, b) => a.y - b.y || a.x - b.x)) {
       tile.w = Math.max(1, Math.min(COLS, tile.w));
       tile.x = Math.max(0, Math.min(COLS - tile.w, tile.x));
+      if (settled.some((s) => overlaps(tile, s))) Object.assign(tile, spotAmong(tile.w, tile.h, settled));
+      settled.push(tile);
       applyRect(tile);
       notifyResize(tile);
     }
@@ -386,6 +394,17 @@ function isFree(rect, excludeId) {
   return isFreeExcept(rect, [excludeId]);
 }
 
+/** First spot, scanning top-to-bottom, left-to-right, clear of these tiles. */
+function spotAmong(w, h, others) {
+  for (let y = 0; y < 400; y += 1) {
+    for (let x = 0; x <= COLS - w; x += 1) {
+      const rect = { x, y, w, h };
+      if (!others.some((o) => overlaps(rect, o))) return { x, y };
+    }
+  }
+  return { x: 0, y: 0 };
+}
+
 /** First free spot scanning top-to-bottom, left-to-right. */
 function findSpot(w, h) {
   for (let y = 0; y < 200; y += 1) {