Quoting aelias@chromium.org from  issue 746286 :

    This is working as intended.  The speed of toggle-autoscroll cannot be controlled when the mouse cursor leaves the main content window, so rather than let the autoscroll keep going out of control, I cancelled the autoscroll in this situation instead.  (You can still leave the window and keep autoscroll going with middle-hold-autoscroll.)

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What does "keep going out of control" mean?
The old behavior was acceptable and not surprising, whereas the new behavior is quite annoying when the browser window is not maximized.
Also, AFAIK, the new behavior breaks historical/standard practices observed in all/most UI at least in Windows.
Since aelias@ is inactive, can someone please reconsider and undo the change?

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Bisect info: 479929 (good) - 479936 (bad)
https://chromium.googlesource.com/chromium/src/+log/0c7f6df4..07711ae8?pretty=fuller
Suspecting r479931 = 5971e47dc08dbb773f58fd6fb13b1df7917fd295 = https://crrev.com/2918053002 by aelias@chromium.org
"Move middle-click autoscroll to synthetic fling."
Landed in 61.0.3133.0

Repro:
1. open the attached test.html
2. resize the browser window to half the screen size
3. middle-click in the top right corner of the page and release the button
4. move the mouse diagonally down and to the right until it's outside the window

Expected: scrolling continues
Observed: scrolling stops

Deleted: test.html 90 bytes

test.html 90 bytes View Download

Able to reproduce the issue on Windows 10 using chrome reported version #64.0.3282.119 and latest canary #66.0.3334.0.
Issue is not applicable to OS-Mac and OS-linux.

As per comment#1, providing the bisect results:
Bisect info: 479929 (good) - 479936 (bad)
https://chromium.googlesource.com/chromium/src/+log/0c7f6df4..07711ae8?pretty=fuller
Suspecting r479931 = 5971e47dc08dbb773f58fd6fb13b1df7917fd295 = https://crrev.com/2918053002 by aelias@chromium.org
"Move middle-click autoscroll to synthetic fling."
Landed in 61.0.3133.0

aelias@ - Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.
ccing Bokan@(reviewer) as the chromium id of author of the issue seems to be inactive.

Thanks...!!

I tend to agree that if Edge and other Windows apps have this behavior then we should preserve it. However, as noted in  issue 746286 , users can still click and hold the middle mouse button and get the expected behavior so I consider this to be low pri.

Sahel, this might be something you could take a look at when you get to moving autoscroll fling gestures into the browser.

JFYI, it works like that in Firefox too. 

Also I personally think the usage "middle-hold-autoscroll" is very rare (among users) compared to the normal "autoscroll", so even it works there, it doesn't quite justify the regression in normal autoscroll.