fix(webui): avoid mobile welcome composer overlap

webui/src/components/thread/ThreadViewport.tsx

@@ -532,7 +532,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         ref={scrollRef}
         className={cn(
           "thread-viewport-scrollbar absolute inset-0 scroll-auto scrollbar-thin",
-          hasMessages && hasVerticalOverflow ? "overflow-y-auto" : "overflow-hidden",
+          hasVerticalOverflow ? "overflow-y-auto" : "overflow-hidden",
           "[&::-webkit-scrollbar]:w-1.5",
           "[&::-webkit-scrollbar-thumb]:rounded-full",
           "[&::-webkit-scrollbar-thumb]:bg-muted-foreground/30",
@@ -574,8 +574,11 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         ) : (
           <div ref={contentRef} className="mx-auto flex min-h-full w-full max-w-[72rem] flex-col px-3 sm:px-4">
             <div className="flex w-full flex-1 flex-col items-center pb-[calc(0.75rem+env(safe-area-inset-bottom))] pt-6 sm:justify-center sm:py-12">
-              <div className="relative flex w-full max-w-[58rem] flex-1 flex-col items-center justify-end gap-5 sm:block sm:flex-none">
-                <div className="pointer-events-none absolute inset-0 flex items-center justify-center sm:pointer-events-auto sm:static sm:block">
+              <div
+                data-testid="thread-welcome-layout"
+                className="relative grid w-full max-w-[58rem] flex-1 grid-rows-[minmax(min-content,1fr)_auto] gap-8 sm:block sm:flex-none"
+              >
+                <div className="flex min-h-0 items-center justify-center sm:absolute sm:inset-x-0 sm:bottom-[calc(100%+2rem)]">
                   {emptyState}
                 </div>
                 <div className="w-full">{composer}</div>

webui/src/tests/thread-viewport.test.tsx

@@ -598,6 +598,77 @@ describe("ThreadViewport", () => {
     }
   });
 
+  it("keeps the welcome composer above a mobile soft keyboard", async () => {
+    const visualViewport = stubVisualViewport({ innerHeight: 800, height: 480 });
+    try {
+      const { container } = render(
+        <ThreadViewport
+          messages={emptyMessages}
+          isStreaming={false}
+          composer={<textarea aria-label="Message input" />}
+          emptyState={<div>welcome</div>}
+          showScrollToBottomButton={false}
+        />,
+      );
+      const scroller = container.querySelector(".thread-viewport-scrollbar") as HTMLElement;
+      const input = screen.getByLabelText("Message input");
+
+      act(() => {
+        input.focus();
+        fireEvent.focusIn(input);
+      });
+
+      await waitFor(() => expect(scroller).toHaveStyle({ bottom: "320px" }));
+    } finally {
+      visualViewport.restore();
+    }
+  });
+
+  it("separates the mobile welcome copy and composer into responsive rows", () => {
+    render(
+      <ThreadViewport
+        messages={emptyMessages}
+        isStreaming={false}
+        composer={<div>composer</div>}
+        emptyState={<div>welcome</div>}
+      />,
+    );
+
+    const layout = screen.getByTestId("thread-welcome-layout");
+    expect(layout).toHaveClass(
+      "grid",
+      "gap-8",
+      "grid-rows-[minmax(min-content,1fr)_auto]",
+    );
+    expect(screen.getByText("welcome").parentElement).toHaveClass("min-h-0");
+    expect(screen.getByText("welcome").parentElement?.className).toContain(
+      "sm:bottom-[calc(100%+2rem)]",
+    );
+  });
+
+  it("allows the welcome view to scroll when a short viewport overflows", async () => {
+    const { container } = render(
+      <ThreadViewport
+        messages={emptyMessages}
+        isStreaming={false}
+        composer={<div>composer</div>}
+        emptyState={<div>welcome</div>}
+        showScrollToBottomButton={false}
+      />,
+    );
+    const scroller = container.querySelector(".thread-viewport-scrollbar") as HTMLElement;
+    Object.defineProperties(scroller, {
+      scrollHeight: { configurable: true, value: 620 },
+      clientHeight: { configurable: true, value: 320 },
+    });
+
+    act(() => {
+      window.dispatchEvent(new Event("resize"));
+    });
+
+    await waitFor(() => expect(scroller).toHaveClass("overflow-y-auto"));
+  });
+
   it("scrolls recent messages into view when the composer receives focus", async () => {
     const scrollTo = vi.fn();
     const { container } = render(