I was making an overlay-component with many background heads and some buttons and the result was this:

if I remove the background from the flow layout:

minecraft_version=1.19.3
owo_version=0.10.2+1.19.3

While this does in fact happen, it's not really a bug as much as it's simply a consequence of how items are rendered. Since they use depth values >0, they show on top - that's just how rendering works. The slight artifacts you're seeing are most likely caused by the fact that not all of owo-ui currently enables depth testing and thus depth buffer writes correctly when rendering.

To fix your immediate issue, you should be able to just give your overlay a sufficiently large Z-Index (~500 should do the trick).

I'll consider adding an always-on-top property similar to what the draggable container provides
Cheers