chore(pixel): tighten scroll_depth README, drop unnecessary cast

- Trim scroll_depth README cell; iframe limitation belongs in
  docs.immutable.com (separate PR) and isn't scroll-specific anyway
- Drop unnecessary (el as HTMLElement) cast — Element.scrollTop is in
  the DOM spec
- Tighten setupAutocapture docstring

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bkbooth

packages/audience/pixel/README.md

@@ -85,7 +85,7 @@ All events fire automatically with no instrumentation required.
 | `session_end` | Page unload (`visibilitychange` / `pagehide`) | `sessionId`, `duration` (seconds) |
 | `form_submitted` | HTML form submission | `formAction`, `formId`, `formName`, `fieldNames`. `emailHash` at `full` consent only. |
 | `link_clicked` | Outbound link click (external domains only) | `linkUrl`, `linkText`, `elementId`, `outbound: true` |
-| `scroll_depth` | Scroll milestone reached (25%, 50%, 75%, 90%, 100%) | `depth` (integer). Fires on both standard document scroll and SPA internal scroll containers (e.g. `overflow: auto` divs) larger than 50% of the viewport. Milestones reset on each `page` call so SPA route changes start fresh. No event fires on pages with no scrollable area at all. **iframe limitation:** a cross-origin iframe cannot be observed from the parent page — install the pixel inside the iframe itself for those integrations. |
+| `scroll_depth` | Scroll milestone reached (25%, 50%, 75%, 90%, 100%) | `depth` (integer). Fires on standard document scroll or on any internal scroll container larger than half the viewport. Milestones reset on each `page` call. |
 
 ### Disabling specific auto-capture
 

packages/audience/pixel/src/autocapture.ts

@@ -117,7 +117,7 @@ function setupScrollTracking(
       if (!el) return;
       const scrollPos = el === document.documentElement
         ? window.scrollY
-        : (el as HTMLElement).scrollTop;
+        : el.scrollTop;
       checkAndFire(el, scrollPos);
     });
   };
@@ -140,10 +140,8 @@ function setupScrollTracking(
 
 /**
  * Attach document-level listeners for form submissions, outbound link clicks,
- * and scroll depth milestones.
- * Returns `{ teardown, resetScroll }` — call `teardown()` to remove all
- * listeners, and `resetScroll()` to clear fired milestones (e.g. on SPA
- * route changes).
+ * and scroll depth milestones. `resetScroll()` clears fired scroll milestones
+ * (call from `Pixel.page()` on SPA route changes).
  *
  * - Single document-level listener per event type (event delegation).
  * - Consent is checked at fire time, not at attach time.