Hi there! And welcome to DashBoard!
As an answer to your question, yes there is!
Every element in DashBoard, be this a button, a text field, a scroll, a fader or a canvas or a tab, they all allign on a layer, and the last thing you created will be on the top layer (unless you right click it and tell it to move down.
Now if you drag and drop something you'll see that you can overlap things and see how they cover eachother up etc.
In your example, if you first select your button, and then move it to the top layer, and move it over the tab, it will show, but it wont hide again if you choose another tab, and it will not follow the tab if you move the tab.
However if you create a button INSIDE the tab now, you'll notice that it appears and hides as you would expect, and that if you move the tab, the button moves with it.
But! No, you dont have to recreate all your buttons etc. Organizing is good!
So, what you do, is mark your button you want to move, then CUT (ctrl+x), then SELECT the background of your tab, and PASTE (ctrl+v), this will put the button in the top left corner of THAT tab, and inside THAT canvas, so it behaves as you expect it to. Now if you take your second element/button, cut that out, and select the button that you placed inside the tab allready, and then paste, you'll notice that it placed it in the top left corner of that element. And there you go!
#DashBoard