UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Steps to reproduce the problem:
I'm using the latest stable Chrome (71.0.3578.98 64bit):

1. Download the attached repl.zip file.
2. Go to chrome://extensions.
3. Drag repl.zip onto this page.
4. Click on the extension icon (should just be a black PNG) to trigger the popup.
5. Wait for the popup to load. It should contain a single credit card field.
6. Click within the credit card field.

What is the expected behavior?
On Firefox, credit card forms on a non-HTTPS site show the following error: "Firefox has detected an insecure site. Form autofill is temporarily disabled." This is great! Plus Chrome does the same thing.

HOWEVER, Firefox provides no such warning on extension popups.

Based on that behavior, I would expect the same to occur here.

What went wrong?
Upon clicking on the credit card form field within the extension popup, the following warning is shown:

"Automatic credit card filling is disabled because this form does not use a secure connection."

The extension's popup.html file embeds an iframe (allowed in the CSP of the manifest) which loads a site via HTTPS. Valid certificate, no mixed content, etc. 

The same error occurs even when the form is included directly in the popup.html file (i.e. without an iframe).

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 71.0.3578.98  Channel: stable
OS Version: OS X 10.14.0
Flash Version: 

Posting a bug on the advice of users on the extensions mailing list.

Relevant thread here:
https://groups.google.com/a/chromium.org/d/msg/chromium-extensions/15GLd0TVYJw/2fmgJBXRDgAJ

On a fresh installation of the latest stable Firefox (64.0.2 64bit), no credit card warnings are shown in the popup. Firefox doesn't enable credit card autofill by default; but if you enable it (https://support.mozilla.org/en-US/questions/1221549#answer-1121952), autofill also works as expected.
 

Deleted: repl.zip 1.4 KB

repl.zip 1.4 KB Download

Deleted: unexpected-behavior.png 19.9 KB

	unexpected-behavior.png 19.9 KB View Download