Fix for OSX auto hiding scrollbars (#103)

* Fix for OSX auto hiding scrollbars
* Move fallback scrollbar size to const

packages/core-scroll/core-scroll.js

@@ -6,6 +6,7 @@ const ATTR = 'data-core-scroll'
 const UUID = `data-${name}-${version}`.replace(/\W+/g, '-') // Strip invalid attribute characters
 const MOVE = {up: {y: -1, prop: 'top'}, down: {y: 1, prop: 'bottom'}, left: {x: -1}, right: {x: 1}}
 const SIGNIFICANT_DRAG_THRESHOLD = 10
+const FALLBACK_SCROLLBAR_SIZE = 18
 const FRICTION = 0.8
 const VELOCITY = 20
 
@@ -23,9 +24,12 @@ export default function scroll (elements, move = '') {
       target.style.willChange = 'scroll-position' // Enhance performace
       target.style.webkitOverflowScrolling = 'touch' // Momentum scoll on iOS
 
-      // Must be after setting overflow scroll
-      const scrollbarWidth = target.offsetWidth - target.clientWidth + 1 // Plus one fixes safari pixelbug
-      const scrollbarHeight = target.offsetHeight - target.clientHeight + 1 // Plus one fixes safari pixelbug
+      // Calculate scrollBareSizes for hiding
+      // Must be after setting overflow:scroll
+      // Default to 18px (max reports scrollbar size 0 when "auto hiding")
+      // Plus one fixes safari pixelbug
+      const scrollbarWidth = (target.offsetWidth - target.clientWidth || FALLBACK_SCROLLBAR_SIZE) + 1
+      const scrollbarHeight = (target.offsetHeight - target.clientHeight || FALLBACK_SCROLLBAR_SIZE) + 1
 
       target.style.maxHeight = `calc(100% + ${scrollbarHeight}px)` // Consistent height
       target.style.marginRight = `-${scrollbarWidth}px`