the preview takes the room, the controls take what they need

Edit put a fixed 420px band above a scrolling column, so the preview was a letterbox in the mode where
changes are actually being made — and there was empty panel below the controls while the thing being
judged was squeezed.

One column now, for both modes, and it never scrolls as a whole. The preview is flex-1, so it is always
as tall as whatever is left. The controls are shrink-0, so they are exactly as tall as their content
and no taller. Edit takes height from the preview and only as much as it needs.

Capped at 55% with their own scroll, because shrink-0 alone would let a short window hand the controls
everything and leave the preview at nothing — the same failure in the other direction.

The frame is also capped at the device's real height. A 390x844 phone in a 1200px panel would otherwise
be drawn as a phone 1200 pixels tall, which is not a phone. Past that it stops growing and the space
stays empty, which is honest: there is no device that shape.

Two modes were two return paths; now switching modes hides a block rather than rendering a different
component.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/LayoutPanel.tsx

@@ -158,24 +158,21 @@ export const LayoutPanel = () => {
   if (!draft) return <div className="p-4 text-xs text-muted-foreground">{error ?? 'Loading…'}</div>;
 
   /*
-    Viewing: the preview and nothing else, filling the panel.
+    ONE layout for both modes: the preview takes the room, the controls take what they need.
 
-    Not the same thing as hiding the controls. The preview is given the whole height, which is what
-    makes View worth having — at edit height a phone is a letterbox, and a phone is the width somebody
-    most wants to look at properly.
+    It used to be two — View returned the preview alone, and Edit put a fixed 420px band above a
+    scrolling column. That made the preview a letterbox in the mode where you are actually changing
+    things, with empty panel below the controls, and it meant switching modes was a different component
+    rather than a smaller one.
+
+    Now the column fills the panel and never scrolls as a whole. The preview is `flex-1`, so it is
+    always as tall as whatever is left; the controls are `shrink-0`, so they are exactly as tall as
+    their content and no taller. Edit takes height from the preview, and only as much as it needs.
   */
-  if (mode === 'view') {
-    return (
-      <div className="flex size-full flex-col p-4">
-        <LayoutPreview port={devPort} fill />
-      </div>
-    );
-  }
-
   return (
-    <div className="flex size-full flex-col gap-4 overflow-auto p-4">
-      {saved && <p className="text-xs text-success">Saved</p>}
-      {error && <p className="text-xs text-destructive">{error}</p>}
+    <div className="flex size-full flex-col gap-3 p-4">
+      {saved && <p className="shrink-0 text-xs text-success">Saved</p>}
+      {error && <p className="shrink-0 text-xs text-destructive">{error}</p>}
 
       {/*
         The REAL page, embedded — not a drawing of it.
@@ -187,6 +184,16 @@ export const LayoutPanel = () => {
       */}
       <LayoutPreview port={devPort} />
 
+      {/*
+        The controls, and only in Edit.
+
+        `shrink-0` so they never give up height to the preview, `max-h-[55%]` so they can never take
+        the whole panel either — on a short window the preview would otherwise be squeezed to nothing,
+        which is the failure this whole change is about. Past that they scroll on their own rather than
+        scrolling the preview off the top.
+      */}
+      {mode === 'edit' && (
+        <div className="flex max-h-[55%] shrink-0 flex-col gap-4 overflow-auto">
       {/* ── Colours ──────────────────────────────────────────────────────────────────────────── */}
       <section className="flex flex-col gap-3 rounded-lg border border-border p-4">
         <h3 className="text-sm font-medium">Colours</h3>
@@ -250,6 +257,8 @@ export const LayoutPanel = () => {
           ))}
         </div>
       </section>
+        </div>
+      )}
     </div>
   );
 };
@@ -273,7 +282,7 @@ export const LayoutPanel = () => {
  * was the other option and it is a guess about where the browser kept it; the platform tried that and
  * says so in its own comments.
  */
-const LayoutPreview = ({ port, fill }: { port: number | null; fill?: boolean }) => {
+const LayoutPreview = ({ port }: { port: number | null }) => {
   const client = useClient();
   /*
     Through a ref, and this is the whole reason the preview hung.
@@ -289,7 +298,7 @@ const LayoutPreview = ({ port, fill }: { port: number | null; fill?: boolean })
   const clientRef = useRef(client);
   clientRef.current = client;
 
-  const { device } = useViewMode();
+  const { device, mode } = useViewMode();
   const { width, height } = DEVICES[device];
 
   const [ready, setReady] = useState(false);
@@ -345,13 +354,21 @@ const LayoutPreview = ({ port, fill }: { port: number | null; fill?: boolean })
   */
   const scale = available.width ? Math.min(1, available.width / width) : 1;
 
-  // In View the frame gets the panel's height; editing, a fixed band, so the controls below stay put
-  // instead of moving every time somebody switches device.
-  const shownHeight = fill ? Math.max(0, available.height) : Math.min(height * scale, 420);
+  /*
+    Whatever room is left, capped at the device's own height.
+
+    The cap matters: a 390x844 phone in a 1200px-tall panel would otherwise be drawn as a phone 1200
+    pixels tall, which is not a phone. Past its real height the frame stops growing and the space stays
+    empty, which is honest — there is no device that shape.
+
+    It was a fixed 420px band in Edit. That made the preview a letterbox in the mode where changes are
+    being made, and left empty panel below the controls.
+  */
+  const shownHeight = Math.max(0, Math.min(available.height, height * scale));
 
   return (
-    <section className={`flex flex-col gap-2 ${fill ? 'min-h-0 flex-1' : ''}`}>
-      <div ref={box} className={`min-h-0 ${fill ? 'flex-1' : ''}`}>
+    <section className="flex min-h-0 flex-1 flex-col gap-2">
+      <div ref={box} className="min-h-0 flex-1">
         <div
           className="mx-auto overflow-hidden rounded-lg border border-border bg-white"
           // The scaled footprint, so the border hugs the frame and the page below does not jump when
@@ -382,8 +399,10 @@ const LayoutPreview = ({ port, fill }: { port: number | null; fill?: boolean })
         </div>
       </div>
 
-      {!fill && (
-        <p className="text-[10px] text-muted-foreground">
+      {/* Only while editing. In View the preview is the whole point, and a caption is a line of height
+          taken from it to say something the device buttons already show. */}
+      {mode === 'edit' && (
+        <p className="shrink-0 text-[10px] text-muted-foreground">
           The development server, live at {width}px. Save to see a change here.
         </p>
       )}