diff --git a/cloakbrowser/human/scroll.py b/cloakbrowser/human/scroll.py
index 4bc7cca..1087ea3 100644
--- a/cloakbrowser/human/scroll.py
+++ b/cloakbrowser/human/scroll.py
@@ -64,6 +64,13 @@ def human_scroll_into_view(
     """
     viewport = page.viewport_size
     if not viewport:
+        # Headed launches default to no_viewport so the page tracks the real OS
+        # window; page.viewport_size is then None. Fall back to the live window
+        # dimensions so humanize works headed (the stealth-relevant mode).
+        viewport = page.evaluate(
+            "() => ({ width: window.innerWidth, height: window.innerHeight })"
+        )
+    if not viewport or not viewport.get("height"):
         raise RuntimeError("Viewport size not available")
 
     viewport_height = viewport["height"]
diff --git a/cloakbrowser/human/scroll_async.py b/cloakbrowser/human/scroll_async.py
index 3ec4894..7bbb981 100644
--- a/cloakbrowser/human/scroll_async.py
+++ b/cloakbrowser/human/scroll_async.py
@@ -60,6 +60,13 @@ async def async_human_scroll_into_view(
     """
     viewport = page.viewport_size
     if not viewport:
+        # Headed launches default to no_viewport so the page tracks the real OS
+        # window; page.viewport_size is then None. Fall back to the live window
+        # dimensions so humanize works headed (the stealth-relevant mode).
+        viewport = await page.evaluate(
+            "() => ({ width: window.innerWidth, height: window.innerHeight })"
+        )
+    if not viewport or not viewport.get("height"):
         raise RuntimeError("Viewport size not available")
 
     viewport_height = viewport["height"]
diff --git a/js/src/human-puppeteer/scroll.ts b/js/src/human-puppeteer/scroll.ts
index 8ef8bab..3108be5 100644
--- a/js/src/human-puppeteer/scroll.ts
+++ b/js/src/human-puppeteer/scroll.ts
@@ -93,8 +93,16 @@ export async function humanScrollIntoView(
   cursorY: number,
   cfg: HumanConfig,
 ): Promise<{ box: ElementBounds; cursorX: number; cursorY: number }> {
-  const viewport = page.viewport();
-  if (!viewport) throw new Error('Viewport size not available');
+  // Headed launches default to null defaultViewport so the page tracks the real
+  // OS window; page.viewport() is then null. Fall back to the live window
+  // dimensions so humanize works headed (the stealth-relevant mode).
+  let viewport = page.viewport();
+  if (!viewport) {
+    viewport = await page.evaluate(
+      () => ({ width: window.innerWidth, height: window.innerHeight }),
+    );
+  }
+  if (!viewport || !viewport.height) throw new Error('Viewport size not available');
 
   let box = await getBox();
   if (!box) throw new Error('Element not found while scrolling into view');
diff --git a/js/src/human/scroll.ts b/js/src/human/scroll.ts
index fe235e9..d959469 100644
--- a/js/src/human/scroll.ts
+++ b/js/src/human/scroll.ts
@@ -53,8 +53,16 @@ export async function humanScrollIntoView(
   cursorY: number,
   cfg: HumanConfig,
 ): Promise<{ box: ElementBounds; cursorX: number; cursorY: number; didScroll: boolean }> {
-  const viewport = page.viewportSize();
-  if (!viewport) throw new Error('Viewport size not available');
+  // Headed launches default to no_viewport so the page tracks the real OS
+  // window; page.viewportSize() is then null. Fall back to the live window
+  // dimensions so humanize works headed (the stealth-relevant mode).
+  let viewport = page.viewportSize();
+  if (!viewport) {
+    viewport = await page.evaluate(
+      () => ({ width: window.innerWidth, height: window.innerHeight }),
+    );
+  }
+  if (!viewport || !viewport.height) throw new Error('Viewport size not available');
 
   let box = await getBox();
   if (!box) throw new Error('Element not found while scrolling into view');