How about allowing D&D on to tabs (like Mac Finder tabs or Chrome tabs): dragging and dropping on to the stack tabs title area (either to bring that window to the top of the stack and/or to drop into that window (which in stacked Tracker tabbed windows would move the file from one window (source folder) to the other window (destination folder).
Would that be hard to implement?
see “Move Files between Tabs” on this page: https://www.takecontrolbooks.com/resources/0184/site/chap06.html
No we didn’t considered a solution without modifier key.
Personally, I think the issue of accidentally stacking or tiling something would result in a bad UX. Furthermore, how would you un-stack and un-tile? I guess to be consistent you would not require a modifier key for this either but how would you move the whole group then? I can imagine that for new user that could become confusing.
Not saying that there isn’t a nice solution though but we haven’t really thought about it at the time…
Hi Clemens!
The forum informs that you’ve been absent for 7 years. Wow! What’s going on? 
My idea was that dragging with the right mouse button would unS&T. But I also agreed that auto-S&T would be confusing to unprepared users.
Therefore, a setting to forego the OPT key and do auto-S&T, as suggested by @Meanwhile, would be best.
I’d prefer to have the window pop to front (after 1/4s or something of hovering) instead of copying/moving blindly. I even filed ticket #15480 for that thinking about it recently.
Oh, seems like X512’s recent change solved the un-stacked scenario of a partially obscured window not popping up. I’ll update the ticket.
Maybe you’re right, but I don’t know if I agree about the bad impression: If I drop a tab directly onto another tab and it stacks them together, I’d think I’d discovered a cool feature, not a problem. How often do you want to place windows directly on top of each other in almost the same position, and in the case where you do, wouldn’t having them stacked be useful?
In the end it’s down to what the devs consider the best default behaviour, and given that the aim is to have defaults that everyone likes over configurability, they might not want to have an option to change the behaviour. But anyway it’s food for thought…
Shake the stack? 
I do love S&T the way they are, but if there is a way to make it more accessible/discoverable it would be great too!
We can implement it for experimenting but I’m fairly sure it will be triggered by accident a lot more than you’d think. You just want to move some window out of the way, you carelessly drag it away, and it stacks with something (and is resized, which is hard to revert).
It’s hard to tell without experimenting, I think. So maybe let’s do that and see what happens?
Good call: You’re right, I have no idea. I’m glad that you think it’s worth pursuing though.
I created ticket #15727 to track this enhancement and hopefully catch the attention of a dev that is interested to give it a try. 
Thanks for adding / modifying that ticket.