fix(webui): soften composer scroll boundary

webui/src/components/thread/ThreadViewport.tsx

@@ -744,8 +744,8 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
               ref={messageRegionRef}
               data-testid="thread-message-region"
               className={cn(
-                "thread-viewport-scrollbar row-start-1 flex min-h-0 min-w-0 flex-col",
-                "scroll-auto justify-start overflow-x-hidden px-3 pb-4 pt-4 sm:px-4",
+                "thread-message-viewport thread-viewport-scrollbar row-start-1 flex min-h-0 min-w-0 flex-col",
+                "scroll-auto justify-start overflow-x-hidden px-3 pb-0 pt-4 sm:px-4",
                 "[overflow-anchor:none] [scrollbar-width:none]",
                 "[&::-webkit-scrollbar]:hidden",
                 hasVerticalOverflow ? "overflow-y-auto" : "overflow-hidden",
@@ -768,6 +768,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
                   onQuoteSelection={onQuoteSelection}
                 />
               </div>
+              <div aria-hidden className="thread-message-end-gap shrink-0" />
               <div ref={bottomRef} aria-hidden className="h-px shrink-0" />
             </div>
           ) : (
@@ -808,7 +809,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
               }}
               className={cn(
                 "row-start-2 z-10 w-full",
-                hasMessages ? "relative bg-background" : "relative self-center",
+                hasMessages ? "thread-composer-dock relative" : "relative self-center",
               )}
             >
               <div

webui/src/globals.css

@@ -377,6 +377,7 @@
    * bottom without a remount or a transform clone.
    */
   .thread-layout {
+    --thread-composer-fade-height: 2.25rem;
     grid-template-rows: minmax(min-content, 1fr) auto 0fr;
     transition: grid-template-rows 220ms ease-out;
   }
@@ -392,6 +393,33 @@
     }
   }
 
+  /**
+   * Keep the transcript and composer visually continuous. The scrollport owns
+   * the fade because overflow clips its contents before an adjacent overlay
+   * can soften the edge. The matching in-flow gap keeps the last line fully
+   * visible once the user reaches the bottom.
+   */
+  .thread-message-viewport {
+    -webkit-mask-image: linear-gradient(
+      to bottom,
+      #000 0,
+      #000 calc(100% - var(--thread-composer-fade-height)),
+      transparent 100%
+    );
+    mask-image: linear-gradient(
+      to bottom,
+      #000 0,
+      #000 calc(100% - var(--thread-composer-fade-height)),
+      transparent 100%
+    );
+  }
+  .thread-message-end-gap {
+    height: var(--thread-composer-fade-height);
+  }
+  .thread-composer-dock {
+    background: hsl(var(--background));
+  }
+
   @keyframes composer-status-strip-enter {
     0% {
       max-height: 0;

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

@@ -259,7 +259,8 @@ describe("ThreadViewport", () => {
     const messageRegion = screen.getByTestId("thread-message-region");
     expect(messageRegion).toHaveClass("justify-start");
     expect(messageRegion).not.toHaveClass("justify-end");
-    expect(messageRegion).toHaveClass("pb-4");
+    expect(messageRegion).toHaveClass("thread-message-viewport");
+    expect(messageRegion).toHaveClass("pb-0");
     expect(messageRegion.className).not.toContain("5rem");
   });
 
@@ -316,7 +317,9 @@ describe("ThreadViewport", () => {
     expect(scroller).not.toContainElement(composerDock);
     expect(scroller.parentElement).toContainElement(composerDock);
     expect(composerDock).toHaveClass("relative");
+    expect(composerDock).toHaveClass("thread-composer-dock");
     expect(composerDock).not.toHaveClass("sticky");
+    expect(scroller.querySelector(".thread-message-end-gap")).toBeInTheDocument();
     expect(scroller.lastElementChild).toHaveClass("h-px", "shrink-0");
   });