This regression may have existed on Sequoia too, but I only saw it on
Tahoe.
The issue is that we should not be setting up titlebar accessory views
when there is no titlebar. This was triggering an AppKit assertion.
To further simplify things, I always use the basic window styling if
window decorations are off, too.
Resolves#7591
This moves our CI to build macOS on Sequoia (macOS 15) with Xcode 26,
including the new macOS 26 beta SDK.
Importantly, this will make our builds on macOS 26 use the new styling.
I've added a new job that ensures we can continue to build with Xcode 16 and
the macOS 15 SDK, as well, although I think that might come to an end
when we switch over to an IconComposer-based icon. I'll verify then. For
now, we continue to support both.
I've also removed our `hasLiquidGlass` check, since this will now always
be true for macOS 26 builds.
