Environment.OSVersion.Version behavior in macOS 26

[vcsjones]

Apple is switching up their versioning numbers again, this time by aligning all of their platforms to be version 26.

Much like Apple Big Sur, Apple returns a compatibility value for software that does not indicate that it understands the new versioning scheme, as was seen in #41012.

Right now, Environment.OSVersion.Version returns 16.0.0 on macOS 26.

Setting the SYSTEM_VERSION_COMPAT environment variable to 0 causes the correct value to be returned, as does building against a newer SDK. With this, the API correctly returns 26.0.0.

This also likely impacts mobile Apple platforms as well, since they are also adopting the new version scheme. They may have a harder time using the environment variable as a work around.

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[akoeplinger]

Looks like we'll need to update

runtime/src/libraries/Common/src/Interop/OSX/Interop.libobjc.cs

Lines 49 to 56 in 8bcc9e2

	if (major == 10 && minor == 16)
	{
	// We get "compat" version for 11.0 unless we build with updated SDK.
	// Hopefully that will be before 11.x comes out
	// For now, this maps 10.16 to 11.0.
	major = 11;
	minor = 0;
	}

[vcsjones]

Yeah, the trick is making sure we do it right for iOS, too, which we didn't have to do last time. I don't know right now what the compatibility number iOS returns is, if it is returning one at all.

[akoeplinger]

#116536 mentions that iOS, tvOS, or Catalyst don't return compatibility numbers so nothing needs to be done for these.