Improve UIKit style masking and button opacity

packages/stack/src/views/Header/Header.js

@@ -684,10 +684,10 @@ const styles = StyleSheet.create({
   },
   iconMask: {
     // These are mostly the same as the icon in ModularHeaderBackButton
-    height: 21,
-    width: 12,
+    height: 23,
+    width: 14.5,
     marginLeft: 8.5,
-    marginTop: -0.5, // resizes down to 20.5
+    marginTop: -2.5,
     alignSelf: 'center',
     resizeMode: 'contain',
     transform: [{ scaleX: I18nManager.isRTL ? -1 : 1 }],

packages/stack/src/views/Header/HeaderStyleInterpolator.js

@@ -172,9 +172,9 @@ function forLeftButton(props) {
   const outputRange = [
     0,
     hasHeader(scenes[first]) ? 0 : 1,
-    hasHeader(scenes[first]) ? 0.1 : 1,
+    hasHeader(scenes[first]) ? 0.3 : 1,
     hasHeader(scenes[index]) ? 1 : 0,
-    hasHeader(scenes[last]) ? 0.1 : 1,
+    hasHeader(scenes[last]) ? 0.3 : 1,
     hasHeader(scenes[last]) ? 0 : 1,
     0,
   ];