Hey sdy@, just took a look at this. I think we could either do what you suggested with the simple heuristic, or we can just change the value stored in tab_strip_drag_controller.mm's |kVertTearDistance| (https://cs.chromium.org/chromium/src/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm?rcl=9e9ab272629494f1b72726e3231c90e107a2fecb&l=21) to make it so you have to drag more vertical distance to tear out.

I have a patch here (https://chromium-review.googlesource.com/c/chromium/src/+/642211) that does both (copying Views behaviour by increasing the vertical tab drag threshold to match Views (I just eyeballed this, Views looks like it will snap off when it reaches the bottom of the omnibox - for reference the code is here https://cs.chromium.org/chromium/src/chrome/browser/ui/views/tabs/tab_drag_controller.cc), as well as make it so that only horizontal tab movement will activate when a max horizontal distance has been travelled in drag mode (I've just set that to an arbitrary value, which seems to work pretty well for me when playing with it, but we can update this value).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6f6bbf52c672eead857752e1f230a513216f2891

commit 6f6bbf52c672eead857752e1f230a513216f2891
Author: Patricia Lor <patricialor@chromium.org>
Date: Fri Sep 08 03:19:57 2017

Mac: Emulate macOS native tab dragging for tearing off tabs.

It's too easy to tear off tabs on Mac. To make this behavior consistent with
native macOS tabs (e.g. on Finder and Safari), this patch will:
 - Tear the tab off after a short drag distance when the user drags straight
   down from the tab strip.
 - Tear the tab off after a long drag distance when the user drags a little
   horizontally, then tries to drag downwards.

Manual test - open up Chrome with a bunch of tabs. Drag one downwards and
observe it makes a new window once the mouse passes the top of the
omnibox. Next, drag a tab horizontally, then drag it down. Observe that it will
not tear off into its own window until reaching somewhere below the top chrome
(most likely inside the web contents). Also check that tearing the tab off the
window area (e.g. moving the tab past the right or left edge of the window) is
not subject to the larger tab drag distance.

Bug:  732133 
Change-Id: I5cde3f2123f88ab9e6622480bb64ce50ba7e0d72
Reviewed-on: https://chromium-review.googlesource.com/642211
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Patti <patricialor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500498}
[modify] https://crrev.com/6f6bbf52c672eead857752e1f230a513216f2891/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm

Verified the fix on Mac 10.12.6 using latest chrome version #63.0.3212.0 as per the comment #0 and #3.
Attaching screen cast for reference.
Observed that tab when dragged horizontally did not tear off into its own window until reaching somewhere below the top chrome.
Hence, the fix is working as expected. 
Adding the verified labels.

Thanks...!!

Deleted: 732133.mp4 1.2 MB

	732133.mp4 1.2 MB View Download

This feels so much better! \o/

Great, thank you very much :-)

Just one question regarding tearing the tab off the
window area (moving the tab past the right or left edge of the window): Are there any plans for a larger tab drag distance in this case? At the moment it is a very short distance (maybe 5px?). Thank you.

+screencast re#c7.

Deleted: screencast.mov 206 KB

screencast.mov 206 KB Download

A bigger horizontal distance sounds reasonable. Since this bug has already been TE-Verified, would you mind filing a new one (and adding it to Hotlist-PlatformExcellence)?

Done:  Issue 763948