fix(chat): status row floats inside the existing bottom spacer, no extra inset

The rest gap after completion doubled up: the status overlay reserved
its own end inset on top of the footer's 10vh spacer. The overlay now
floats within the spacer zone — its height feeds only the glide target
(the live line still rests above it), the list gets no extra inset, and
completion cannot shift layout because there is nothing to collapse.
The finished message's metadata lands in the slack the glide left above
the status row, restoring the old morph-in-place feel. The frozen
height hack and the opaque background are gone — the overlay only ever
covers blank spacer, not text.

packages/ui/src/components/chat/ChatContainer.tsx

@@ -159,7 +159,6 @@ type ChatViewportProps = {
     anchorMessageId: string | null;
     onAnchorReady: (messageId: string, anchorIndex: number) => void;
     onAnchorSizeChanged: (messageId: string) => void;
-    composerOverlayHeight: number;
     onIsAtEndChange: (isAtEnd: boolean) => void;
     onTimelineDataChange: () => void;
     pendingRevealWork: boolean;
@@ -204,7 +203,6 @@ const ChatViewport = React.memo(({
     anchorMessageId,
     onAnchorReady,
     onAnchorSizeChanged,
-    composerOverlayHeight,
     onIsAtEndChange,
     onTimelineDataChange,
     pendingRevealWork,
@@ -407,7 +405,10 @@ const ChatViewport = React.memo(({
                     anchorMessageId={anchorMessageId}
                     onAnchorReady={onAnchorReady}
                     onAnchorSizeChanged={onAnchorSizeChanged}
-                    composerOverlayHeight={composerOverlayHeight}
+                    // Zero end inset: the footer spacer already reserves the
+                    // zone the floating status row covers; adding its height
+                    // again produced a double-tall blank band at rest.
+                    composerOverlayHeight={0}
                     onIsAtEndChange={onIsAtEndChange}
                     onTimelineDataChange={onTimelineDataChange}
                     listHeader={listHeader}
@@ -423,7 +424,7 @@ const ChatViewport = React.memo(({
                 <div className="pointer-events-none absolute inset-x-0 bottom-0 z-10">
                     <div
                         ref={onStatusOverlayNode}
-                        className="pointer-events-auto bg-background pb-2 [&:not(:has(*))]:hidden"
+                        className="pointer-events-auto pb-2 [&:not(:has(*))]:hidden"
                     >
                         <StatusRowContainer />
                     </div>
@@ -932,9 +933,6 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({
     const [statusOverlayHeight, setStatusOverlayHeight] = React.useState(0);
     const composerOverlayHeight = statusOverlayHeight;
     const statusOverlayObserverRef = React.useRef<ResizeObserver | null>(null);
-    React.useEffect(() => {
-        setStatusOverlayHeight(0);
-    }, [currentSessionKey]);
     const onStatusOverlayNode = React.useCallback((node: HTMLDivElement | null) => {
         statusOverlayObserverRef.current?.disconnect();
         statusOverlayObserverRef.current = null;
@@ -944,10 +942,7 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({
         }
         const update = () => {
             const height = node.getBoundingClientRect().height;
-            // Hold the reserved height when the status row empties: collapsing
-            // it at end-of-stream shifts the settled content. It resets only
-            // on session change.
-            setStatusOverlayHeight((prev) => (height > prev + 1 ? height : prev));
+            setStatusOverlayHeight((prev) => (Math.abs(prev - height) < 1 ? prev : height));
         };
         const observer = new ResizeObserver(update);
         observer.observe(node);
@@ -1374,7 +1369,6 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({
                 anchorMessageId={anchorMessageId}
                 onAnchorReady={onAnchorReady}
                 onAnchorSizeChanged={onAnchorSizeChanged}
-                composerOverlayHeight={composerOverlayHeight}
                 onIsAtEndChange={onIsAtEndChange}
                 onTimelineDataChange={onTimelineDataChange}
                 messageListRef={messageListRef}