When I compiled my legacy project with Tahoe's macOS 26 SDK, NSRulerViews are showing a very different design:
Under prior macOS versions the horizontal and verrical ruler's background were blurring the content view, which was extending under the rulers, showing through their transparency.
With Tahoe the horizontal ruler is always reflecting the scrollview's background color, showing the blurred content view beneath.
And the vertical ruler is always completely transparent (without any blurring), showing the content together with the ruler's markers and ticks.
It's difficult to describe, I'll try to replicate this behavior with a minimal test project, and probably file a bug report / enhancement request.
But before I take next steps, can anyone confirm this observation? Maybe it is an intentional design decision by Apple?