Lose focus when .focus() is called while the element already has the focus

[caoxiaoyi03]

Description

The paper-input element appears to lose its focus when .focus() is called while it already has the focus.

Expected outcome

Calling .focus() on paper-input when it's already in focus should keep the focus in the iron-input component.

Actual outcome

While paper-input appears to still have the focus (.focused === true), the iron-input inside is not (therefore, editing is not possible) and the appearance indicates out of focus.

Steps to reproduce

1. Put a paper-input element in the page.
2. Open the page in a web browser.
3. Use some delaying mechanism (window.setTimeout or window.setInterval) to queue a .focus() call after some time.
4. Focus on the paper-input element before the .focus() call triggers.
5. Wait for the .focus() call to trigger. The paper-input element will appear to have lost its focus.

Live demo

This issue can be demonstrated with any live page with a paper-input element (for example, this page on WebComponents) by running the following code in console:

window.setInterval(() => {
  document.querySelector('paper-input').focus();
  console.log(document.querySelector('paper-input').focused);
}, 5000);

After the code above was run, whenever the first paper-input element on the page gets the focus, it will be gone during the next cycle in no more than 5 seconds. Console log will show that the .focused property of the paper-input is still true.

Browsers Affected

• Chrome
• Firefox
• Safari 9 (not tested)
• Safari 8 (not tested)
• Safari 7 (not tested)
• Edge
• IE 11 (not tested due to not supporting ES6 features in the testing code)
• IE 10 (not tested)

[caoxiaoyi03]

While checking .focused before calling .focus() can be a workaround for this issue in most cases, this workaround will not prevent cases in other libraries (which should not expect .focus() to have such a behavior). For example, when using iron-dropdown's focusTarget property to link to a paper-input (I suppose .focus() is called in iron-dropdown's code).