In order to trigger this you need to open a tab and get it to unmount before the idle callback happens.

In those cases, it's creating a memory leak.

[Tracking Requested - why for this release]: There is a small potential memory leak caused by https://bugzilla.mozilla.org/show_bug.cgi?id=1576223 which is in 70, so we probably want an uplift for this.

https://hg.mozilla.org/mozilla-central/rev/eba9b9f96abc

I have verified this issue with the steps provided in Comment 2 on Latest Firefox Nightly 71.0a1 (Build ID: 20190915214245) on Windows 10 x64, Mac 10.14.5 and Arch Linux 4.16.6 x64.
No error about setState on unmounted dscard are displayed in the console when toggling on and off the browser.newtabpage.activity-stream.discoverystream.enabled preference

Comment on attachment 9093637 [details]
Bug 1579480 - uplift newtab DS cards not removing idle callback

Beta/Release Uplift Approval Request

• User impact if declined: Potential memory leak, but otherwise an error message.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: Managed to get better steps to test.

1. Set browser.newtabpage.activity-stream.debug to true
2. Open a new tab
3. Open about:config
4. Toggle browser.newtabpage.activity-stream.discoverystream.enabled on and off, a few times.
5. Check console.

Expected, no errors about setState on unmounted dscard. "Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method."

• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Verified on nightly, small change, and has unit tests.
• String changes made/needed: none

Comment on attachment 9093637 [details]
Bug 1579480 - uplift newtab DS cards not removing idle callback

Fixes a possible memory leak. Approved for 70.0b8.

Comment on attachment 9093637 [details]
Bug 1579480 - uplift newtab DS cards not removing idle callback

Whoops, missed that this was still waiting on r+ in Phabricator.

Scott, can you try to find a reviewer in the next couple of days so this can make it into beta 10 on Thursday? Thanks!

Comment on attachment 9093637 [details]
Bug 1579480 - uplift newtab DS cards not removing idle callback

Fix for memory leak, OK for uplift for beta 10.

Should this be landed on integration too ?

https://hg.mozilla.org/releases/mozilla-beta/rev/870f8e24fea4

(In reply to Andreea Pavel [:apavel] from comment #11)

Should this be landed on integration too ?

No, that landed in more extensive way in comment 3.

I have verified this issue with the steps provided in Comment 6 on Latest Firefox Beta 70.0b10 (Build ID: 20190926005616) on Windows 10 x64, Mac 10.14.6 and Arch Linux 4.16.6 x64.
No error about setState on unmounted dscard are displayed in the console when toggling on and off the browser.newtabpage.activity-stream.discoverystream.enabled preference.