Accordion: Handle `box-sizing: border-box` in animations

Fixes #9264
Closes gh-1287
Closes gh-1459
(cherry picked from commit 4b017b4)

ui/accordion.js

@@ -516,6 +516,7 @@ return $.widget( "ui.accordion", {
 		var total, easing, duration,
 			that = this,
 			adjust = 0,
+			boxSizing = toShow.css( "box-sizing" ),
 			down = toShow.length &&
 				( !toHide.length || ( toShow.index() < toHide.index() ) ),
 			animate = this.options.animate || {},
@@ -558,7 +559,9 @@ return $.widget( "ui.accordion", {
 				step: function( now, fx ) {
 					fx.now = Math.round( now );
 					if ( fx.prop !== "height" ) {
-						adjust += fx.now;
+						if ( boxSizing === "content-box" ) {
+							adjust += fx.now;
+						}
 					} else if ( that.options.heightStyle !== "content" ) {
 						fx.now = Math.round( total - toHide.outerHeight() - adjust );
 						adjust = 0;