Hi Ben, we want this only for dragging app windows? or browser windows (tab dragging or drag from caption area) too?

Since drag a browser window from overview to above a snapped window will also trigger the preview area.

The problem is that, drag a browser window to above the snapped window will trigger the scrim if drag exceed 40% of the screen height.

Feel weird if to show both of them at same time. I feel if we want to support the preview area we'd better to remove the scrim for browser window in this case. WDYT?

See the recorded video,
https://drive.google.com/file/d/0B5I0jFeLxqIiQXk3aXRqRDZranFqLWUxeTFiUTBzM2ZyRGYw/view?usp=sharing

Hey Min, good point. We should show the preview area for dragged tabs when a drop in that position would create a new window. Right now that's not syonymous with when the scrim shows up, but it was supposed to be - and the idea was actually to have the two overlaid. We may soon get rid of the scrim and the ability to create new windows anyway as we've received a bunch of feedback that people are creating windows too easily and accidentally.

Tab dragging of a snapped browser window or drag a snapped app window to above the snapped window will have different behaviors.
For app window,
1. show the preview area doesn't matter the position of the dragged window, it is only needed to be above the snapped window.

For tab dragged browser window,
1. In the caption area, should merge the dragged window into the snapped window. No preview area 
   is needed in this case.
2. Outside the caption area, show the preview area.
3. If the window has been dragged further than 40% of the height of the display from the top, 
   show the scrim at the same time. Then, we can see both preview area and scrim in this case.

Per offline chat with Ben, we think it is not so good to make app dragging and tab dragging have too much different behaviors here.

I will defer this here until we have better solution for this. Thanks.