Fixed disappearance of header shadow on iOS

src/assets/javascripts/components/tabs/index.ts

@@ -77,7 +77,7 @@ export function mountTabs(
           if (screen) {
             return watchViewportFrom(el, { header$, viewport$ })
               .pipe(
-                paintHideable(el, 20),
+                paintHideable(el, 10),
                 map(hidden => ({ hidden }))
               )
 

src/assets/javascripts/observables/agent/viewport/offset/index.ts

@@ -46,8 +46,8 @@ export interface ViewportOffset {
  */
 export function getViewportOffset(): ViewportOffset {
   return {
-    x: pageXOffset,
-    y: pageYOffset
+    x: Math.max(0, pageXOffset),
+    y: Math.max(0, pageYOffset)
   }
 }