fix(floatinglabel): Estimate hidden scroll width

packages/mdc-dom/README.md

@@ -34,6 +34,7 @@ Function Signature | Description
 --- | ---
 `closest(element: Element, selector: string) => ?Element` | Returns the ancestor of the given element matching the given selector (which may be the element itself if it matches), or `null` if no matching ancestor is found.
 `matches(element: Element, selector: string) => boolean` | Returns true if the given element matches the given CSS selector.
+`estimateScrollWidth(element: Element) => number`  | Returns the true optical width of the element if visible or an estimation if hidden by a parent element with `display: none;`.
 
 ### Event Functions
 

packages/mdc-dom/ponyfill.ts

@@ -47,3 +47,30 @@ export function matches(element: Element, selector: string): boolean {
       || element.msMatchesSelector;
   return nativeMatches.call(element, selector);
 }
+
+/**
+ * Used to compute the estimated scroll width of elements. When an element is
+ * hidden due to display: none; being applied to a parent element, the width is
+ * returned as 0. However, the element will have a true width once no longer
+ * inside a display: none context. This method computes an estimated width when
+ * the element is hidden or returns the true width when the element is visble.
+ * @param {Element} element the element whose width to estimate
+ */
+export function estimateScrollWidth(element: Element): number {
+  // Check the offsetParent. If the element inherits display: none from any
+  // parent, the offsetParent property will be null (see
+  // https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetParent).
+  // This check ensures we only clone the node when necessary.
+  const htmlEl = element as HTMLElement;
+  if (htmlEl.offsetParent !== null) {
+    return htmlEl.scrollWidth;
+  }
+
+  const clone = htmlEl.cloneNode(true) as HTMLElement;
+  clone.style.setProperty('position', 'absolute');
+  clone.style.setProperty('transform', 'translate(-9999px, -9999px)');
+  document.documentElement.appendChild(clone);
+  const scrollWidth = clone.scrollWidth;
+  document.documentElement.removeChild(clone);
+  return scrollWidth;
+}

packages/mdc-floating-label/component.ts

@@ -22,6 +22,7 @@
  */
 
 import {MDCComponent} from '@material/base/component';
+import {estimateScrollWidth} from '@material/dom/ponyfill';
 import {MDCFloatingLabelAdapter} from './adapter';
 import {MDCFloatingLabelFoundation} from './foundation';
 
@@ -59,7 +60,7 @@ export class MDCFloatingLabel extends MDCComponent<MDCFloatingLabelFoundation> {
     const adapter: MDCFloatingLabelAdapter = {
       addClass: (className) => this.root_.classList.add(className),
       removeClass: (className) => this.root_.classList.remove(className),
-      getWidth: () => this.root_.scrollWidth,
+      getWidth: () => estimateScrollWidth(this.root_),
       registerInteractionHandler: (evtType, handler) => this.listen(evtType, handler),
       deregisterInteractionHandler: (evtType, handler) => this.unlisten(evtType, handler),
     };

packages/mdc-floating-label/package.json

@@ -24,6 +24,7 @@
   "dependencies": {
     "@material/animation": "^4.0.0",
     "@material/base": "^4.0.0",
+    "@material/dom": "^4.0.0",
     "@material/feature-targeting": "^4.0.0",
     "@material/rtl": "^4.0.0",
     "@material/theme": "^4.0.0",

packages/mdc-textfield/test/component.test.ts

@@ -284,7 +284,7 @@ describe('MDCTextField', () => {
     const component = new MDCTextField(root);
     const adapter = (component.getDefaultFoundation() as any).adapter_;
     expect(adapter.hasClass('foo')).toBe(false);
-    expect(adapter.getLabelWidth()).toEqual(0);
+    expect(adapter.getLabelWidth()).toBeGreaterThan(0);
     expect(() => adapter.floatLabel).not.toThrow();
   });
 

test/unit/mdc-dom/ponyfill.test.js

@@ -25,7 +25,7 @@ import {assert} from 'chai';
 import bel from 'bel';
 import td from 'testdouble';
 
-import {closest, matches} from '../../../packages/mdc-dom/ponyfill.ts';
+import {closest, matches, estimateScrollWidth} from '../../../packages/mdc-dom/ponyfill.ts';
 
 suite('MDCDom - ponyfill');
 
@@ -86,3 +86,19 @@ test('#matches supports vendor prefixes', () => {
   assert.isTrue(matches({webkitMatchesSelector: () => true}, ''));
   assert.isTrue(matches({msMatchesSelector: () => true}, ''));
 });
+
+test('#estimateScrollWidth returns the default width when the element is not hidden', () => {
+  const root = bel`<span>
+    <span id="i0" style="width:10px;"></span>
+  </span>`;
+  const el = root.querySelector('#i0');
+  assert.strictEqual(estimateScrollWidth(el), 10);
+});
+
+test('#estimateScrollWidth returns the estimated width when the element is hidden', () => {
+  const root = bel`<span style="display:none;">
+    <span id="i0" style="width:10px;"></span>
+  </span>`;
+  const el = root.querySelector('#i0');
+  assert.strictEqual(estimateScrollWidth(el), 10);
+});