GeezerVison

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

• A custom property whose var() cannot be resolved is stored as core's unsetValue sentinel. When another variable reads it, evaluation calls .trim() on the sentinel and throws, which aborts styling of the view. A stylesheet with a chain of forward references (--a: var(--b) before --b, as Bootstrap writes its variables) renders the app blank.
• var() fallbacks are cut at the first ) and the first ,, so var(--x, rgb(0, 0, 255)) and comma lists such as gradient stops are truncated.
• Custom properties are only resolved in declaration order.
• A value that throws when applied (e.g. var() resolving to an invalid color) leaves the previously applied value in place.

What is the new behavior?

• var() finds its matching closing parenthesis, and the fallback is everything after the first comma, as the spec requires.
• A custom property set to initial uses the var() fallback.
• Custom properties are settled before the properties that use them, and re-evaluated when one refers to another declared after it.
• A custom property that cannot be resolved holds null, CSS's guaranteed-invalid value: var() takes its fallback and the parent's value does not show through. Properties in a cycle resolve the same way. This is the crash fix.
• A declaration that is invalid at computed-value time computes as unset instead of keeping the previous value (css-values-5).

Tests: css-variable-expression.spec.ts and style-scope.spec.ts (forward-reference chains, cycles, an unresolvable variable not inheriting the parent's value, invalid values unsetting). The chain and cycle tests reproduce the TypeError without the fix. The apps/automated expectation for var(--undefined-var, var(--my-fallback-var), yellow) changes from lime to unset, which is what browsers do (lime, yellow is not a color). The automated suite passes on an Android emulator (API 36) with this branch merged with the other CSS PRs.

Verified with all of these core PRs merged together (#11527–#11537): apps/automated passes on the iOS 26.4 simulator (1830 OK, 0 failed) and on an Android API 36 emulator (1692 OK; the one failure is the network-dependent IMAGE-CACHE.test_ImageCache_ValidUrl timeout, which also passes on reruns).