diff --git a/packages/chess/src/ui/PerInstancePanel.tsx b/packages/chess/src/ui/PerInstancePanel.tsx
index 1199fa0..be7d5b4 100644
--- a/packages/chess/src/ui/PerInstancePanel.tsx
+++ b/packages/chess/src/ui/PerInstancePanel.tsx
@@ -56,13 +56,23 @@ export function PerInstancePanel({
   const [selectedSquare, setSelectedSquare] = useState<string | null>(null);
 
   // No layout bound — show the empty state prompt.
+  // Per-instance modifiers are square-bound (b1, e4, etc.), so we
+  // can't render a board preview or accept clicks until a layout
+  // supplies the pieces. The prompt + button focus the layout picker
+  // in the subhead so the user can bind one in a single click.
   if (boundLayout === null) {
     return (
       <div
         data-testid="per-instance-panel"
         className="flex flex-1 flex-col items-center justify-center gap-4 p-8"
       >
-        <p className="text-sm text-neutral-500 italic">Select a layout first</p>
+        <div className="max-w-sm text-center space-y-2">
+          <p className="text-sm text-neutral-500 italic">Select a layout first</p>
+          <p className="text-xs text-neutral-400 leading-relaxed">
+            Per-piece modifiers are tied to specific squares. Bind a starting
+            layout to pick which pieces get modifiers.
+          </p>
+        </div>
         <button
           data-testid="per-instance-choose-layout"
           onClick={onLayoutSelect}
@@ -95,29 +105,44 @@ export function PerInstancePanel({
   }
 
   return (
-    <div data-testid="per-instance-panel" className="flex flex-1 overflow-hidden">
-      {/* Board preview */}
-      <div className="flex-1 flex items-center justify-center p-4 bg-white">
-        <LayoutBoardView
-          pieces={boundLayout.pieces}
-          selectedSquare={selectedSquare}
-          testId="per-instance-board"
-          onSquareClick={handleSquareClick}
-        />
+    <div
+      data-testid="per-instance-panel"
+      className="flex flex-1 flex-col overflow-hidden bg-white"
+    >
+      {/* Board preview — stacked on top and centered so it gets the
+          full column width (no more sharing with a side detail panel
+          that squeezes it to a tiny square). Scroll-locked. */}
+      <div className="flex-1 min-h-0 flex items-center justify-center p-4 overflow-auto">
+        <div className="w-full max-w-md">
+          <LayoutBoardView
+            pieces={boundLayout.pieces}
+            selectedSquare={selectedSquare}
+            testId="per-instance-board"
+            onSquareClick={handleSquareClick}
+          />
+        </div>
       </div>
 
-      {/* Square detail sidebar */}
-      <aside className="w-72 border-l border-neutral-200 bg-neutral-50 p-4 overflow-y-auto space-y-4">
+      {/* Square detail strip — stacked BELOW the board instead of
+          beside it. Fixed-height, scrolls its own overflow. Keeps
+          this column to ONE visual lane so the modal no longer
+          renders four cramped columns side-by-side. */}
+      <div
+        className={`border-t border-neutral-200 bg-neutral-50 shrink-0 ${
+          selectedSquare === null ? 'py-3 px-4' : 'p-4'
+        } max-h-[45%] overflow-y-auto`}
+      >
         {selectedSquare === null ? (
-          <p className="text-sm text-neutral-500 italic">
+          <p className="text-xs text-neutral-500 italic text-center">
             Click a piece on the board to add modifiers.
           </p>
         ) : (
-          <>
-            {/* Selected square header */}
-            <div className="flex items-start justify-between">
-              <div>
-                <h3 className="text-xs font-bold text-neutral-500 uppercase tracking-widest mb-1">
+          <div className="space-y-3">
+            {/* Selected square header — horizontal so it's compact
+                inside the bottom strip. */}
+            <div className="flex items-center justify-between gap-3 flex-wrap">
+              <div className="flex items-baseline gap-3">
+                <h3 className="text-xs font-bold text-neutral-500 uppercase tracking-widest">
                   Square {selectedSquare}
                 </h3>
                 {selectedPiece !== undefined && (
@@ -126,7 +151,7 @@ export function PerInstancePanel({
                   </p>
                 )}
               </div>
-              <div className="flex flex-col items-end gap-1">
+              <div className="flex items-center gap-1">
                 {squareModifiers.length > 0 && (
                   <button
                     type="button"
@@ -137,7 +162,7 @@ export function PerInstancePanel({
                         entries: squareModifiers.map(({ modifier }) => {
                           const { square: _sq, ...rest } = modifier;
                           return rest;
-                        })
+                        }),
                       });
                     }}
                     className="px-2 py-0.5 text-[10px] font-bold uppercase tracking-wider text-blue-600 bg-blue-50 border border-blue-200 rounded hover:bg-blue-100 transition-colors"
@@ -151,7 +176,12 @@ export function PerInstancePanel({
                     data-testid="paste-instance-modifiers"
                     onClick={() => {
                       if (clipboard.kind === 'instance-modifiers') {
-                        onPaste(clipboard.entries.map(entry => ({ ...entry, square: selectedSquare })));
+                        onPaste(
+                          clipboard.entries.map((entry) => ({
+                            ...entry,
+                            square: selectedSquare,
+                          })),
+                        );
                       }
                     }}
                     className="px-2 py-0.5 text-[10px] font-bold uppercase tracking-wider text-green-600 bg-green-50 border border-green-200 rounded hover:bg-green-100 transition-colors"
@@ -162,35 +192,48 @@ export function PerInstancePanel({
               </div>
             </div>
 
-            {/* Existing modifiers list */}
-            {squareModifiers.length > 0 && (
-              <ul data-testid="instance-modifiers-list" className="space-y-1">
-                {squareModifiers.map(({ modifier, index }) => (
-                  <li
-                    key={index}
-                    data-testid={`instance-modifier-${selectedSquare}-${modifier.kind}`}
-                    className="flex items-center gap-2 px-3 py-2 bg-white border border-neutral-200 rounded text-sm"
+            {/* Horizontal band: existing modifier chips on the left,
+                add-form on the right. Wraps to stacked on narrow
+                widths via flex-wrap. */}
+            <div className="flex flex-wrap gap-3 items-start">
+              <div className="flex-1 min-w-[240px] space-y-1">
+                {squareModifiers.length > 0 ? (
+                  <ul
+                    data-testid="instance-modifiers-list"
+                    className="space-y-1"
                   >
-                    <span className="flex-1 font-medium text-neutral-700 break-all">
-                      {modifier.kind}: {JSON.stringify(modifier.value)}
-                    </span>
-                    <button
-                      onClick={() => onDelete(index)}
-                      aria-label={`Delete ${modifier.kind} modifier`}
-                      className="shrink-0 text-red-500 hover:text-red-700 text-xs font-bold leading-none"
-                    >
-                      ✕
-                    </button>
-                  </li>
-                ))}
-              </ul>
-            )}
-
-            {/* Add modifier form */}
-            <AddModifierForm square={selectedSquare} onAdd={onAdd} />
-          </>
+                    {squareModifiers.map(({ modifier, index }) => (
+                      <li
+                        key={index}
+                        data-testid={`instance-modifier-${selectedSquare}-${modifier.kind}`}
+                        className="flex items-center gap-2 px-3 py-1.5 bg-white border border-neutral-200 rounded text-xs"
+                      >
+                        <span className="flex-1 font-medium text-neutral-700 break-all">
+                          {modifier.kind}: {JSON.stringify(modifier.value)}
+                        </span>
+                        <button
+                          onClick={() => onDelete(index)}
+                          aria-label={`Delete ${modifier.kind} modifier`}
+                          className="shrink-0 text-red-500 hover:text-red-700 text-xs font-bold leading-none"
+                        >
+                          ✕
+                        </button>
+                      </li>
+                    ))}
+                  </ul>
+                ) : (
+                  <p className="text-[11px] text-neutral-400 italic py-1">
+                    No modifiers on this square yet.
+                  </p>
+                )}
+              </div>
+              <div className="w-full sm:w-[280px] shrink-0">
+                <AddModifierForm square={selectedSquare} onAdd={onAdd} />
+              </div>
+            </div>
+          </div>
         )}
-      </aside>
+      </div>
     </div>
   );
 }
@@ -309,7 +352,7 @@ function AddModifierForm({
   }
 
   return (
-    <div className="space-y-2 border-t border-neutral-200 pt-3">
+    <div className="space-y-2">
       <h4 className="text-xs font-bold text-neutral-500 uppercase tracking-widest">
         Add Modifier
       </h4>