full screen ?

Tutorial:

scirra.com/tutorials/73/supporting-multiple-screen-sizes

Set scaling mode to "Scale outer".

Set "Unbounded scrolling=yes" in all layouts.

Add 10-20% more background to all layouts.

I found a possible workaround when I was making a a mobile game to support the different device ratios. Because all tablets and phones have different ratios....

Set full screen, no letter boxes.

Now, create a rectangle, this rectangle will be invisible and on HUD layer.

Set origin points on the rectangle for where all of your HUD elements need to spawn.

Set the invisible rectangle to be the size of the viewport.

Have the rectangle spawn all of your HUD elements on it's origin points

Now all of your hud elements will spawn appropriately no matter the device.

Now you can experiment with different scaling for the active game layer to get your game the right size.

Some devices will see more or a little less of the game play area, but your HUD and everything else will be placed nicely regardless.

— The same can be done much easier with Anchor behavior.

Yes, anchor works great if you are actively resizing the window during gameplay, which isn't true for 99% of games.