Correctly handle box-shadows

Now box-shadows rects are calculated the same way as in Blink
engine.

lib/element-rect.js

@@ -27,39 +27,52 @@ exports.get = function get(element) {
         .then(function(rect) {
             return element.getCssValue('box-shadow').then(function(boxShadow) {
                 var shadows = parseBoxShadow(boxShadow);
-                return shadows.reduce(addBoxShadowRect, rect);
+                return adjustRect(rect, shadows);
             });
         });
 };
 
 function parseBoxShadow(value) {
-    var regex = /.+? ((?:\d*)(?:\.\d+)?)px ((?:\d*)(?:\.\d+)?)px ((?:\d*)(?:\.\d+)?)px ((?:\d*)(?:\.\d+)?)px( inset)?/,
+    var regex = /.+? ([-+]?\d*\.?\d+)px ([-+]?\d*\.?\d+)px ([-+]?\d*\.?\d+)px ([-+]?\d*\.?\d+)px( inset)?/,
         results = [],
         match;
 
     while ((match = value.match(regex))) {
-        //ignore inset shadows
-        if (!match[5]) {
-            results.push({
-                offsetX: +match[1],
-                offsetY: +match[2],
-                blurRadius: +match[3],
-                spreadRadius: +match[4]
-            });
-        }
-
+        results.push({
+            offsetX: +match[1],
+            offsetY: +match[2],
+            blurRadius: +match[3],
+            spreadRadius: +match[4],
+            inset: !!match[5]
+        });
         value = value.substring(match.index + match[0].length);
     }
     return results;
 }
 
-function addBoxShadowRect(rect, shadow) {
-    var size = shadow.blurRadius + shadow.spreadRadius,
-        shadowRect = new Rect(
-            rect.x + shadow.offsetX - size,
-            rect.y + shadow.offsetY - size,
-            rect.width - shadow.offsetX + 2 * size,
-            rect.height - shadow.offsetY + 2 *size
-        );
-    return rect.merge(shadowRect);
+function adjustRect(rect, shadows) {
+    var extent = calculateShadowExtent(shadows);
+    return new Rect(
+        Math.max(0, rect.x + extent.left),
+        Math.max(0, rect.y + extent.top),
+        rect.width - extent.left + extent.right,
+        rect.height - extent.top + extent.bottom
+    );
+}
+
+function calculateShadowExtent(shadows) {
+    var result = {top: 0, left: 0, right: 0, bottom: 0};
+    shadows.forEach(function(shadow) {
+        if (shadow.inset) {
+            //skip inset shadows
+            return;
+        }
+
+        var blurAndSpread = shadow.spreadRadius + shadow.blurRadius;
+        result.left = Math.min(shadow.offsetX - blurAndSpread, result.left);
+        result.right = Math.max(shadow.offsetX + blurAndSpread, result.right);
+        result.top = Math.min(shadow.offsetY - blurAndSpread, result.top);
+        result.bottom = Math.max(shadow.offsetY + blurAndSpread, result.bottom);
+    });
+    return result;
 }