The threshold to drop a window into overview is 50% of the distance between the top of the screen and the top of the first row of overview windows now, this is based on fact that overview is opened.

But this is not true when trying to drag a tab out from its source window (drag a tab out from a browser window with multiple windows).
In this case, overview will not be opened behind the dragged window. The source window is behind the dragged window instead. Then,

1. We can never able to drop he dragged window into overview (overview is not opened).
2. We are not able to show the indicator based on the new threshold, since overview is not opened, we are not able to get the top of the first row of overview windows in this case.

Ben ans Kejia, and idea about this? 

Good point, tabs is a special case. In this case where we don't show overview, show the indicators right away as they are now. The feedback from UI review was that they were showing up too easily with apps, for this tabs case I think it's desirable to have them show up right away.

Ben, I think
new_threshold =  0.5 * top of first row of overview windows is not always larger than
old_threshold = 0.1 * screen height.

It must depends on how many windows be opened in overview.
e.g. open at least 4 windows (or at least 2 rows of items in overview) in meowth in landscape, new_threshold = 78 and old_threshold = 83.

And in many of the cases, these two thresholds should don't have too much difference.

Do we need to choose a new threshold for this?

Hi Ben and Kejia, I will defer this until we have a better threshold. Since the new one is not too good to avoid the case that too easy to show the indicators. see #3.