I'm used to paste some stuff in newtab searchbar in using middle button. But it doesn't work anymore.
With the help of mozregression, I found:

6:34.09 INFO: Last good revision: 48b2aa8dc68fa2c5a5edef5fd171267361b4646a
6:34.09 INFO: First bad revision: 5990f24b16efd56b80acc265ef70548481f47784
6:34.09 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=48b2aa8dc68fa2c5a5edef5fd171267361b4646a&tochange=5990f24b16efd56b80acc265ef70548481f47784

which indicates that patch for bug 1516272 could be the one introducing this regression.

:Mardak, could you investigate please ?

This is most likely from the search handoff feature where in-content search just redirects to the address bar.

What exactly is the expected functionality? Does it just paste the contents into the box or does it also do a search at the same time?

If the former, maybe we should somehow handoff on paste events too?

In the mean time, a workaround is to set browser.newtabpage.activity-stream.improvesearch.handoffToAwesomebar=false

As far as I know, it just pastes the contents from the clipboard.

Ah, well middle-click won't quite work as it's implemented right now as it's just a div that looks like a text box. If we want to support this functionality, it could probably be done with a transparent input/text box that can accept these types of events.

Otherwise, I suppose users would learn to middle-click directly into the address bar instead of the in-content one… ?

[Tracking Requested - why for this release]:
Recent regression in nightly

Landed on master in github:
https://github.com/mozilla/activity-stream/commit/f1967e12e85efe919530876d88964040209e1e1b

Export is pending.

https://hg.mozilla.org/mozilla-central/rev/226b130b6360

I have verified that the issue is no longer reproducible on the latest Nightly 66.0a1 (Build ID 20190116093310) on Windows 10 x64, Mac 10.13, and Arch Linux 4.14.3.