As for the original topic of round corners, what if I just tell Windows to round the corners? Could make it a skin setting like Glass?
I guess that is what markf2748 was aiming for, in part?
Example of Modern Cards Dark on a darkish and light background. It works fine as long as the image does not use transparency itself (as Windows stops the rounding then to avoid clashing)
The skin is entirely unmodified here (for testing i'm just always applying it, ultimately it would get a simple condition in the skin)
(1) Based on your 2 screenshots in this post, which display anti-aliasing transparency pixels "outside adjacent" to the round corners, I would expect this to work nicely. Presumably, as you say, the new skin setting could be easily applied to any skin that itself does not draw round corners and does not invoke MC Transparency via
GLASS::Data:Enabled. Potentially a really nice option which addresses the Subject Title of this thread and should satisfy a lot of people. Looking forward to trying it.
I hope this solution will leave the Window control buttons fully customizable in the skin, as now. Note that I have seen several third party Win11 apps which appear to be using MS rounded corners but have nice small window control buttons. Is that also a Microsoft setting?
(2) This thread also evolved to reflect my interest in having anti-aliasing pixels when the MC skin controls the corner rounding radius, without the special Microsoft settings and constraints for (1). Since @ET has shown that
GLASS::Data:Enabled applies to both top and bottom borders (i.e. includes all 4 main window corners), it would be beneficial to have MC transparency
without blur. With that, and using rounded corner opaque artwork for those regions, we can then have a graphics drawing program (such as Inkscape, Gimp, or others) automatically add the anti-aliasing transparency pixels into a png file. Those transparency pixels will presumably be rendered correctly by the skin and should provide smooth round corners in the absence of blur. Not exactly intuitive, but I would expect it to work for the general rounded corner problem, adding design flexibility.
Even better would be full control of blur on an Entry by Entry basis for each bitmap, which is effective when the bitmap has transparency, i.e. a new Attribute.

Thanks.