I see. As I said, we are not really doing anything. And AFAIK there is no opt-in option, Apple should just use the standard style for window. I understand that Tahoe uses (very) different corner radii for different window options, which I personally think is a bad idea. Perhaps the reason you don't see the larger radius in Skim is because we move the toolbar below the titlebar rather than next to the title (because we typically have quite a large toolbar). You could test that out using a hidden SKTitleAndToolbarStyle preference you can set (see the Wiki for details).
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We don't draw the window frame, that is all hidden stuff in AppKit.
BTW the screenshot of Skim does look exactly like in older macOS versions, so I am also confused about what you are asking.
Thanks for your responses and apologies for the confusion.
I was hoping to pre-empt any changes to the existing app .
The screenshot shows a portion of another app (Notebook) which has been updated to conform with Tahoe’s design language.
I see. As I said, we are not really doing anything. And AFAIK there is no opt-in option, Apple should just use the standard style for window. I understand that Tahoe uses (very) different corner radii for different window options, which I personally think is a bad idea. Perhaps the reason you don't see the larger radius in Skim is because we move the toolbar below the titlebar rather than next to the title (because we typically have quite a large toolbar). You could test that out using a hidden SKTitleAndToolbarStyle preference you can set (see the Wiki for details).