It used to work with Chrome 64.0.3282.140

Bisect info: 526470 (good) - 526476 (bad)
https://chromium.googlesource.com/chromium/src/+log/c6e321c6..eadeea9d?pretty=fuller
Suspecting r526475 = f2d2fe87028de36a489f7db3f5fb28da2e9d9b2b = https://crrev.com/c/758236 by jochen@chromium.org
"Use navigation for <a download>"
Landed in 65.0.3310.0

The only workaround I know of is to use download inside a temporary iframe:
  var iframe = Object.assign(document.createElement('iframe'), {
    onload() {
      var doc = this.contentDocument;
      var a = Object.assign(doc.createElement('a'), {
        href: 'data:text/plain;base64,' + btoa('Lorem ipsum dolor sit amet'),
        download: 'data-uri-base-64.txt',
      });
      doc.body.appendChild(a);
      a.dispatchEvent(new MouseEvent('click'));
      setTimeout(() => this.remove());
    },
    style: 'display: none',
  });
  document.body.appendChild(iframe);

Thanks, woxxom

Able to reproduce the issue on reported chrome version 65.0.3325.146, on the latest stable 65.0.3325.162 and on the latest canary 67.0.3368.0 using Windows 10, Ubuntu 14.04 and Mac 10.13.1. 

Bisect Info:
==============
Last Good Build: 65.0.3309.0
First Bad Build: 65.0.3310.0

Suspecting the same from the change log provided in comment#2
https://chromium.googlesource.com/chromium/src/+/f2d2fe87028de36a489f7db3f5fb28da2e9d9b2b
Review URL: https://chromium-review.googlesource.com/758236

@Jochen Eisinger: Please help in assigning it to others if this is not related to your change.

Note: Adding RB-Stable as this is a recent break, Please feel free to remove if not the case. Removing label Needs-Bisect label.

Thanks!

while this should be easy to fix, I don't think this justifies merging a fix back without sufficient bake time, so I'm dropping the releaseblock label.

I'm also dropping interop, as there's no interop issue here (extensions are chrome only)

Related  bug 822542 .

fix is pending reviews btw, but getting reviews during this time of the year is slow :/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/71fdb27d0bf6f3493ee68b2236ff624320fad7e7

commit 71fdb27d0bf6f3493ee68b2236ff624320fad7e7
Author: Jochen Eisinger <jochen@chromium.org>
Date: Tue Mar 20 10:52:28 2018

Allow downloads again from extension views

Now that downloads initially look like navigations, we have to allow
some navigations from extension views such as popups

BUG= 821219 , 823639 
R=clamy@chromium.org,rdevlin.cronin@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I2276a5649120a17974bf576791f74e3eac3d4e3a
Reviewed-on: https://chromium-review.googlesource.com/960003
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544324}
[modify] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/browser/extensions/api/extension_action/browser_action_apitest.cc
[modify] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/browser/extensions/extension_view_host.cc
[add] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/test/data/extensions/api_test/browser_action/popup_download/chromium.png
[add] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/test/data/extensions/api_test/browser_action/popup_download/manifest.json
[add] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/test/data/extensions/api_test/browser_action/popup_download/popup.html
[add] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/chrome/test/data/extensions/api_test/browser_action/popup_download/popup.js
[modify] https://crrev.com/71fdb27d0bf6f3493ee68b2236ff624320fad7e7/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

Thank you, jochen!

Pls request a merge to M66 if cl listed at #11 is safe to merge to M66. Thank you.

Adding "RBS" for M66 for tracking the merge just in case if needed. 

it should work in 66, have you verified?

Ah, my bad, thanks, it does work in Chrome 66.