fix IE11 scrollbar pixel rounding problem

src/util.js

@@ -238,12 +238,13 @@ function getContentRect(el, origin) {
 // NOTE: should use clientLeft/clientTop, but very unreliable cross-browser.
 function getScrollbarWidths(el) {
 	var leftRightWidth = el.innerWidth() - el[0].clientWidth; // the paddings cancel out, leaving the scrollbars
-	var widths = {
-		left: 0,
-		right: 0,
-		top: 0,
-		bottom: el.innerHeight() - el[0].clientHeight // the paddings cancel out, leaving the bottom scrollbar
-	};
+	var bottomWidth = el.innerHeight() - el[0].clientHeight; // "
+	var widths;
+
+	leftRightWidth = sanitizeScrollbarWidth(leftRightWidth);
+	bottomWidth = sanitizeScrollbarWidth(bottomWidth);
+
+	widths = { left: 0, right: 0, top: 0, bottom: bottomWidth };
 
 	if (getIsLeftRtlScrollbars() && el.css('direction') == 'rtl') { // is the scrollbar on the left side?
 		widths.left = leftRightWidth;
@@ -256,6 +257,15 @@ function getScrollbarWidths(el) {
 }
 
 
+// The scrollbar width computations in getScrollbarWidths are sometimes flawed when it comes to
+// retina displays, rounding, and IE11. Massage them into a usable value.
+function sanitizeScrollbarWidth(width) {
+	width = Math.max(0, width); // no negatives
+	width = Math.round(width);
+	return width;
+}
+
+
 // Logic for determining if, when the element is right-to-left, the scrollbar appears on the left side
 
 var _isLeftRtlScrollbars = null;