diff --git a/apps/web/src/components/chat/ComposerBanner.tsx b/apps/web/src/components/chat/ComposerBanner.tsx
index 90da8940d49..9afedb93930 100644
--- a/apps/web/src/components/chat/ComposerBanner.tsx
+++ b/apps/web/src/components/chat/ComposerBanner.tsx
@@ -56,7 +56,11 @@ function Surface({
           : "[--chat-composer-attachment-overlap:0px] before:rounded-[1rem]",
         "before:pointer-events-none before:absolute before:inset-0 before:-z-1 before:border before:border-(--chat-composer-attached-outline)",
         "before:bg-[color-mix(in_srgb,var(--chat-composer-attached-surface)_var(--glass-opacity),transparent)] before:bg-[linear-gradient(var(--chat-composer-attached-tint),var(--chat-composer-attached-tint))] before:backdrop-blur-(--glass-blur) before:backdrop-saturate-(--glass-saturation)",
-        "before:mask-[linear-gradient(to_top,transparent_0_var(--chat-composer-attachment-overlap),black_var(--chat-composer-attachment-overlap))] before:shadow-[0_12px_28px_-18px_rgb(0_0_0/40%)] dark:before:shadow-[0_14px_32px_-18px_rgb(0_0_0/75%)]",
+        // The mask cut-off bleeds one pixel past the seam: Chromium drops the last
+        // device-pixel row of a filtered backdrop when the cut-off lands off the
+        // device-pixel grid, and the composer's surface starts exactly there. The
+        // composer's own glass covers the extra row, so the overlap never shows.
+        "before:mask-[linear-gradient(to_top,transparent_0_calc(var(--chat-composer-attachment-overlap)-1px),black_calc(var(--chat-composer-attachment-overlap)-1px))] before:shadow-[0_12px_28px_-18px_rgb(0_0_0/40%)] dark:before:shadow-[0_14px_32px_-18px_rgb(0_0_0/75%)]",
         "dark:supports-[(backdrop-filter:blur(1px))_or_(-webkit-backdrop-filter:blur(1px))]:before:bg-[linear-gradient(var(--chat-composer-attached-tint),var(--chat-composer-attached-tint)),linear-gradient(to_top,transparent_0_var(--chat-composer-attachment-overlap),rgb(0_0_0/18%)_var(--chat-composer-attachment-overlap),transparent_calc(var(--chat-composer-attachment-overlap)+10px))]",
         "not-supports-[((backdrop-filter:blur(1px))_or_(-webkit-backdrop-filter:blur(1px)))]:before:bg-(--chat-composer-attached-surface)",
         className,