I'm afraid, the SDK docs (C++ and Python) could need some updating to reflect the latest changes done to dialog borders in S26. It seems most flags are pretty useless, now. BORDER_ROUND doesn't seem to do anything anymore, for example.
Of course, I'd prefer to get back the old border behavior, but I'm not naive and think, this could be an options. But at least the changed behavior should be documented.
Thank you for reaching out to us and pointing out these flaws. We are aware of these changes but since it was for quite some time unclear if these changes were intentional or not, we postponed documenting them. The changes to the look and functionalities of GeDialog borders will be included in an upcoming documentation. But since these changes effectively demand symbols or even function arguments to be deprecated, this would be more a developer's task than a documentation task.
I can of course put a note into the documentation, but this would only be a band aid. I will try to find a clean solution by poking the relevant devs.