fix: don't mutate elmStack

lib/commons/color/get-background-color.js

@@ -125,6 +125,7 @@ function includeMissingElements(elmStack, elm) {
 	const tagArray = elmStack.map((elm) => {
 		return elm.tagName;
 	});
+	let bgNodes = elmStack;
 	for (let candidate in elementMap) {
 		if (elementMap.hasOwnProperty(candidate)) {
 			// tagName matches key
@@ -134,39 +135,43 @@ function includeMissingElements(elmStack, elm) {
 					// found an ancestor not in elmStack, and it overlaps
 					let overlaps = axe.commons.dom.visuallyOverlaps(elm.getBoundingClientRect(), ancestorMatch);
 					if (overlaps) {
-						elmStack.splice(elmStack.indexOf(elm) + 1, 0, ancestorMatch);
+						bgNodes.splice(elmStack.indexOf(elm) + 1, 0, ancestorMatch);
 					}
 				}
 			}
 			// tagName matches value
 			// (such as LABEL, when matching itself. It should be in the list, but Phantom skips it)
 			if (elm.tagName === elementMap[candidate] && tagArray.indexOf(elm.tagName) === -1) {
-				elmStack.splice(tagArray.indexOf(candidate) + 1, 0, elm);
+				bgNodes.splice(tagArray.indexOf(candidate) + 1, 0, elm);
 			}
 		}
 	}
+	return bgNodes;
 }
 
 /**
  * Look at document and body elements for relevant background information
  * @private
  * @param {Array} elmStack
  */
-function consultDocumentBody(elmStack) {
+function sortPageBackground(elmStack) {
 	let bodyIndex = elmStack.indexOf(document.body);
 
+	let bgNodes = elmStack;
+
 	if (// Check that the body background is the page's background
 		bodyIndex > 1 && // only if there are negative z-index elements
 		!elmHasImage(document.documentElement) &&
 		getBgColor(document.documentElement).alpha === 0
 	) {
 		// Remove body and html from it's current place
-		elmStack.splice(bodyIndex, 1);
-		elmStack.splice( elmStack.indexOf(document.documentElement), 1);
+		bgNodes.splice(bodyIndex, 1);
+		bgNodes.splice( elmStack.indexOf(document.documentElement), 1);
 
 		// Put the body background as the lowest element
-		elmStack.push(document.body);
+		bgNodes.push(document.body);
 	}
+	return bgNodes;
 }
 
 /**
@@ -190,10 +195,9 @@ color.getBackgroundStack = function(elm) {
 			window.innerHeight - 1);
 
 	let elmStack = document.elementsFromPoint(x, y);
-	includeMissingElements(elmStack, elm);
+	elmStack = includeMissingElements(elmStack, elm);
 	elmStack = dom.reduceToElementsBelowFloating(elmStack, elm);
-
-	consultDocumentBody(elmStack);
+	elmStack = sortPageBackground(elmStack);
 
 	// Return all elements BELOW the current element, null if the element is undefined
 	let elmIndex = elmStack.indexOf(elm);