fix: handle contrast of multiline inline el's

lib/checks/color/color-contrast.json

@@ -13,6 +13,8 @@
         "bgOverlap": "Element's background color could not be determined because it is overlapped by another element",
         "fgAlpha" : "Element's foreground color could not be determined because of alpha transparency",
         "elmPartiallyObscured": "Element's background color could not be determined because it's partially obscured by another element",
+        "elmPartiallyObscuring": "Element's background color could not be determined because it partially overlaps other elements",
+        "outsideViewport": "Element's background color could not be determined because it's outside the viewport",
         "equalRatio": "Element has a 1:1 contrast ratio with the background",
         "default": "Unable to determine contrast ratio"
       }

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

@@ -178,19 +178,15 @@ function sortPageBackground(elmStack) {
 	}
 	return bgNodes;
 }
-
 /**
- * Get all elements rendered underneath the current element, In the order they are displayed (front to back)
- * @method getBackgroundStack
+ * Get coordinates for an element's client rects or bounding client rect
+ * @method getCoords
  * @memberof axe.commons.color
  * @instance
- * @param {Element} elm
- * @return {Array}
+ * @param {DOMRect} rect
+ * @return {Object}
  */
-color.getBackgroundStack = function(elm) {
-	// allows inline elements spanning multiple lines to be evaluated
-	let multipleRects = elm.getClientRects();
-	let rect = multipleRects.length > 1 ? multipleRects[0] : elm.getBoundingClientRect();
+color.getCoords = function(rect) {
 	let x, y;
 	if (rect.left > window.innerWidth) {
 		return;
@@ -205,7 +201,95 @@ color.getBackgroundStack = function(elm) {
 			Math.ceil(rect.top + (rect.height / 2)),
 			window.innerHeight - 1);
 
-	let elmStack = Array.from(document.elementsFromPoint(x, y));
+	return {x, y};
+};
+/**
+ * Get elements from point for block and inline elements, excluding line breaks
+ * @method getRectStack
+ * @memberof axe.commons.color
+ * @instance
+ * @param {Element} elm
+ * @return {Array}
+ */
+color.getRectStack = function(elm) {
+	let boundingCoords = color.getCoords(elm.getBoundingClientRect());
+	if (boundingCoords) {
+		// allows inline elements spanning multiple lines to be evaluated
+		let rects = Array.from(elm.getClientRects());
+		let boundingStack = Array.from(document.elementsFromPoint(boundingCoords.x, boundingCoords.y));
+		if (rects && rects.length > 1) {
+	 		let filteredArr = rects.filter((rect) => {
+				// exclude manual line breaks in Chrome/Safari
+				return rect.width && rect.width > 0;
+			})
+			.map((rect) => {
+				let coords = color.getCoords(rect);
+				if (coords) {
+					return Array.from(document.elementsFromPoint(coords.x, coords.y));
+				}
+			});
+			// add bounding client rect stack for comparison later
+			filteredArr.splice(0, 0, boundingStack);
+			return filteredArr;
+		} else {
+			return [boundingStack];
+		}
+	}
+	return null;
+};
+/**
+ * Get filtered stack of block and inline elements, excluding line breaks
+ * @method filteredRectStack
+ * @memberof axe.commons.color
+ * @instance
+ * @param {Element} elm
+ * @return {Array}
+ */
+color.filteredRectStack = function(elm) {
+	let rectStack = color.getRectStack(elm);
+	if (rectStack && rectStack.length === 1) {
+		// default case, elm.getBoundingClientRect()
+		return rectStack[0];
+	}
+	else if (rectStack && rectStack.length > 1) {
+		let boundingStack = rectStack.shift();
+		let isSame;
+		// iterating over arrays of DOMRects
+		rectStack.forEach((rectList, index) => {
+			if (index === 0) { return; }
+			// if the stacks are the same, use the first one. otherwise, return null.
+			let rectA = rectStack[index - 1],
+				rectB = rectStack[index];
+
+			// if elements in clientRects are the same
+			// or the boundingClientRect contains the differing element, pass it
+			isSame = rectA.every(function(element, elementIndex) {
+				return element === rectB[elementIndex];
+			}) || boundingStack.includes(elm);
+		});
+		if (!isSame) {
+			axe.commons.color.incompleteData.set('bgColor', 'elmPartiallyObscuring');
+			return null;
+		}
+		// pass the first stack if it wasn't partially covered
+		return rectStack[0];
+	} else {
+		// rect outside of viewport
+		axe.commons.color.incompleteData.set('bgColor', 'outsideViewport');
+		return null;
+	}
+};
+/**
+ * Get all elements rendered underneath the current element, In the order they are displayed (front to back)
+ * @method getBackgroundStack
+ * @memberof axe.commons.color
+ * @instance
+ * @param {Element} elm
+ * @return {Array}
+ */
+color.getBackgroundStack = function(elm) {
+	let elmStack = color.filteredRectStack(elm);
+	if (elmStack === null) { return null; }
 	elmStack = includeMissingElements(elmStack, elm);
 	elmStack = dom.reduceToElementsBelowFloating(elmStack, elm);
 	elmStack = sortPageBackground(elmStack);

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

@@ -192,6 +192,39 @@ describe('color.getBackgroundColor', function () {
 		assert.deepEqual(bgNodes, [target]);
 	});
 
+	it('should return a bgcolor for a multiline inline element fully covering the background', function () {
+		fixture.innerHTML = '<div style="position:relative;">' +
+			'<div style="background-color:rgba(0,0,0,1);position:absolute;width:300px;height:200px;"></div>' +
+			'<p style="position: relative;z-index:1;">Text oh heyyyy <a href="#" id="target">and here\'s <br>a link</a></p>' +
+		'</div>';
+		var actual = axe.commons.color.getBackgroundColor(document.getElementById('target'), []);
+		assert.isNotNull(actual);
+		assert.equal(Math.round(actual.blue), 0);
+		assert.equal(Math.round(actual.red), 0);
+		assert.equal(Math.round(actual.green), 0);
+	});
+
+	it('should return null for a multiline block element not fully covering the background', function () {
+		fixture.innerHTML = '<div style="position:relative;">' +
+			'<div id="background" style="background-color:rgba(0,0,0,1); position:absolute; width:300px; height:20px;"></div>' +
+			'<p style="position:relative; z-index:1; width:300px;" id="target">Text content Text content Text content '+
+				'Text content Text content Text content</p>' +
+		'</div>';
+		var actual = axe.commons.color.getBackgroundColor(document.getElementById('target'), []);
+		assert.isNull(actual);
+		assert.equal(axe.commons.color.incompleteData.get('bgColor'), 'elmPartiallyObscured');
+	});
+
+	it('should return null if a multiline inline element does not fully cover background', function () {
+		fixture.innerHTML = '<div style="position:relative;">' +
+			'<div style="background-color:rgba(0,0,0,1);position:absolute;width:300px;height:20px;"></div>' +
+			'<p style="position: relative;z-index:1;">Text oh heyyyy <a href="#" id="target">and here\'s <br>a link</a></p>' +
+		'</div>';
+		var actual = axe.commons.color.getBackgroundColor(document.getElementById('target'), []);
+		assert.isNull(actual);
+		assert.equal(axe.commons.color.incompleteData.get('bgColor'), 'elmPartiallyObscuring');
+	});
+
 	it('should count a TR as a background element for TD', function () {
 		fixture.innerHTML = '<div style="background-color:#007acc;">' +
 		'<table style="width:100%">' +