Thanks Nicki,
I'm not sure what that means to attach a UIWidget at the collider level. I couldn't see a depth setting on the UI Widget.
I have:
UI Root (3d)
Camera at 0,0, -700
FullScreenButton at 0,0,0 - UIWidget achored to UI Root (3d), Box Collider (auto-adjust to widget), UIButton.
Menu Sprite
Other Labels and buttons
Since my web and PC builds are a lower resolution, I'm wondering if the z size of the collider is maybe scaling to less than 0 somehow (that made more sense in my head before I wrote it).
Quick test confirms it also works at Z size of 0 as long as auto-adjust is turned off.
Edited to add: The Depth value of the Widget, of course sorry. That is set to 10, with other buttons within the menu set higher. All above the values of other game elements.