I started writing this in response to @BKMuir over here but then realized it was way off-topic to the original issue so making this new thread.
I tried using the Zazzles ELEMENTS shape of a triangle yesterday.
The masking worked (made the fill empty so I could see the image to be masked, made the outline red) But I needed to move the image - had to un-mask and then the triangle outline disappeared. It still worked, I just had to guess where it was withing the square OUTLINE when I clicked on the mask area.
Not very user friendly ...
Zazzle's shape elements work a bit differently for masks than your own svg's do.
Typically, masking ignores transparent pixels and only reveals the parts of the underlying image that are under the solid pixels of the mask (Yes the mask element could be under or over the image depending on program and it works either way in the Design Tool but I am saying 'under' here for ease of writing, plus in the Design Tool there is a benefit to making the mask element the upper layer.)
Here is an example using an SVG I made. It's just an outline shape with a transparent fill. Same as if you added a shape Element with trans fill plus a border.
Once you mask it, the underlying image shows through where the solid pixels are in the mask. When you enter the new Edit Mask mode, your shape is outlined in white and you can drag the background image under it around the canvas, resize it, rotate it ... You do not have to unmask it and juggle layers around to see what you are doing.
On the other hand, if you use an Element to add a trans shape with border outline like above, it works opposite and treats just the transparent fill area as the mask. Edit Mask mode works the same here but if you Unmask it, your border has been re-set to 0 so you don't see anything on the canvas as the fill is transparent and the solid border is now gone.
Important note about adding a border to an Element you're using as a mask: I get that before, using a trans fill + solid border was probably helpful in seeing what you were doing. But with the new Edit Mask mode that is no longer necessary and a border is actually going to throw things off a bit because - while you can see the border before masking, the masking function ignores it and only uses the trans fill area. So if you made a thick border for your own visuals, the masked area is actually going to be smaller than that so if you need to be really precise, that border could be a problem.
In my experimenting there was no difference between using a solid-filled shape Element or a transparent-fill shape Element when masking and using Edit Mask mode. Not sure this is making any sense so far but my recommendation is, if you use an Element as a mask, make it a solid color with no border to avoid complications.
Back to Edit Mask mode in general ... When I first played with this a few nights ago, I was able to easily move/resize both the mask object and the underlying image independently of each other after entering Edit Mask mode, without having to unmask. I thought this was FABULOUS. I don't think I have published anything with a mask, just because it's such a PITA to set up. So I was super stoked about this new feature. But now tonight, no matter what I try, I cannot move/resize the mask object independently unless I unmask. 😞 I KNOW I was not imagining things or remembering wrong, so either that (being able to move/resize the mask object independently without unmasking) was a glitch or this current state (not being able to do so) is. And this brings me back to what I mentioned way above - if you have to unmask, this is where it is beneficial to have the mask object as the upper layer or otherwise you obviously won't be able to see it unless you juggle layers around.
I think this new feature is really great and it's inspired me to want to actually implement masking in designs, but would be better if it goes back to also being able to also move/resize the mask object independently. But if you place & size the mask object first where it needs to be to work with your overall design, then add the image to be masked and mask them, there's actually not much need to go back and unmask to edit the mask object.
