Cleaner code.

src/service/viewport/viewport-binding-def.js

@@ -1,4 +1,3 @@
-import {Services} from '../../services';
 import {computedStyle} from '../../style';
 import {isExperimentOn} from '../../experiments';
 
@@ -209,18 +208,12 @@ export class ViewportBindingDef {
  * Returns the margin-bottom of the last child of `element` with non-zero
  * height, if any. Otherwise, returns 0.
  *
- * As of Safari 12.1.1, the getBoundingClientRect().height does not include the
- * bottom margin of children and there's no other API that does.
- *
  * @param {!Window} win
  * @param {!Element} element
  * @return {number}
  */
 export function marginBottomOfLastChild(win, element) {
-  if (
-    !isExperimentOn(win, 'bottom-margin-in-content-height') ||
-    !Services.platformFor(win).isSafari()
-  ) {
+  if (!isExperimentOn(win, 'margin-bottom-in-content-height')) {
     return 0;
   }
   let n = element.lastElementChild;

src/service/viewport/viewport-binding-ios-embed-sd.js

@@ -406,20 +406,26 @@ export class ViewportBindingIosEmbedShadowRoot_ {
     const content = this.wrapper_;
     const rect = content./*OK*/ getBoundingClientRect();
 
-    // The Y-position of any element can be offset by the vertical margin
+    // The Y-position of `content` can be offset by the vertical margin
     // of its first child, and this is _not_ accounted for in `rect.height`.
-    // This "top gap" causes smaller than expected contentHeight, so calculate
-    // and add it manually. Note that the "top gap" includes any padding-top
-    // on ancestor elements and the scroller's border-top.
-    const topGapPlusPaddingAndBorder = rect.top + this.getScrollTop();
-    const bottomGap = marginBottomOfLastChild(this.win, this.win.document.body);
+    // This causes smaller than expected content height, so add it manually.
+    // Note this "top" value already includes padding-top of ancestor elements
+    // and getBorderTop().
+    const top = rect.top + this.getScrollTop();
+
+    // As of Safari 12.1.1, the getBoundingClientRect().height does not include
+    // the bottom margin of children and there's no other API that does.
+    const childMarginBottom = marginBottomOfLastChild(
+      this.win,
+      this.win.document.body
+    );
 
     const style = computedStyle(this.win, content);
     return (
-      rect.height +
-      topGapPlusPaddingAndBorder +
+      top +
       parseInt(style.marginTop, 10) +
-      bottomGap +
+      rect.height +
+      childMarginBottom +
       parseInt(style.marginBottom, 10)
     );
   }

src/service/viewport/viewport-binding-ios-embed-wrapper.js

@@ -253,17 +253,20 @@ export class ViewportBindingIosEmbedWrapper_ {
     const content = this.win.document.body;
     const {height} = content./*OK*/ getBoundingClientRect();
 
-    // Unlike viewport-binding-natural.js, there's no need to calculate the
-    // "top gap" since the wrapped body accounts for the top margin of children.
-    // However, the parent's paddingTop and "bottom gap" must be added...
-    const bottomGap = marginBottomOfLastChild(this.win, content);
+    // Unlike other viewport bindings, there's no need to include the
+    // rect top since the wrapped body accounts for the top margin of children.
+    // However, the parent's padding-top (this.paddingTop_) must be added.
+
+    // As of Safari 12.1.1, the getBoundingClientRect().height does not include
+    // the bottom margin of children and there's no other API that does.
+    const childMarginBottom = marginBottomOfLastChild(this.win, content);
 
     const style = computedStyle(this.win, content);
     return (
-      height +
-      this.paddingTop_ +
       parseInt(style.marginTop, 10) +
-      bottomGap +
+      this.paddingTop_ +
+      height +
+      childMarginBottom +
       parseInt(style.marginBottom, 10)
     );
   }

src/service/viewport/viewport-binding-natural.js

@@ -210,19 +210,26 @@ export class ViewportBindingNatural_ {
     // there's no workable analog for either ios-embed-* modes.
     const content = this.getScrollingElement();
     const rect = content./*OK*/ getBoundingClientRect();
-    const style = computedStyle(this.win, content);
-    // The Y-position of any element can be offset by the vertical margin
+
+    // The Y-position of `content` can be offset by the vertical margin
     // of its first child, and this is _not_ accounted for in `rect.height`.
-    // This "top gap" causes smaller than expected contentHeight, so calculate
-    // and add it manually. Note that the "top gap" includes any padding-top
-    // on ancestor elements, and the "bottom gap" remains unaddressed.
-    const topGapPlusPadding = rect.top + this.getScrollTop();
-    const bottomGap = marginBottomOfLastChild(this.win, content);
+    // This causes smaller than expected content height, so add it manually.
+    // Note this "top" value already includes padding-top of ancestor elements
+    // and getBorderTop().
+    const top = rect.top + this.getScrollTop();
+
+    // As of Safari 12.1.1, the getBoundingClientRect().height does not include
+    // the bottom margin of children and there's no other API that does.
+    const childMarginBottom = Services.platformFor(this.win).isSafari()
+      ? marginBottomOfLastChild(this.win, content)
+      : 0;
+
+    const style = computedStyle(this.win, content);
     return (
-      rect.height +
-      topGapPlusPadding +
+      top +
       parseInt(style.marginTop, 10) +
-      bottomGap +
+      rect.height +
+      childMarginBottom +
       parseInt(style.marginBottom, 10)
     );
   }