Hi Dredok, we’re thinking about it.
Having a single texture at editor time is not a desirable workflow. We don’t want users manually making their own atlases, and needing to specify all the UV coordinates when it’s an automatable process. It needs to be improved for UMG to handle UTextures (We do it already for raw pngs for Slate). The goal is to hopefully generate atlases at cook time in whatever format is needed. We may have to have a setting, or maybe there’s an “Atlas” file that you don’t ever really manage, you just specify a bunch of UTextures that it needs to hold and we take packing and compressing it from there. I also don’t want users choosing to organize or pack an atlas when a computer could be better at it. All too often users will break up their atlases based on commonalities that may have nothing to do with efficiency of the atlas for runtime purposes. Additionally some platforms handle differently sized textures better, for example, I think if it’s under 1024x1024 you can use the faster and less precise texture sampler in the shader on some mobile platforms. If the cooker got to choose the atlas size on a per platform basis it could make those kinds of optimization decisions.
If you’d like to author all UI components in a single texture, I’ve seen a UI artist here setup a Photoshop so that named regions could be written out as separate textures. So there’s no reason your source art need be independent textures.