Guard against recursive theme key lookup

Author: thecrypticace

packages/tailwindcss-language-service/src/util/rewriting/var-fallbacks.ts

@@ -3,14 +3,25 @@ import { resolveVariableValue } from './lookup'
 import { replaceCssVars } from './replacements'
 
 export function replaceCssVarsWithFallbacks(state: State, str: string): string {
+  let seen = new Set<string>()
+
   return replaceCssVars(str, {
     replace({ name, fallback }) {
       // Replace with the value from the design system first. The design system
       // take precedences over other sources as that emulates the behavior of a
       // browser where the fallback is only used if the variable is defined.
       if (state.designSystem && name.startsWith('--')) {
+        // TODO: This isn't quite right as we might skip expanding a variable
+        // that should be expanded
+        if (seen.has(name)) return null
         let value = resolveVariableValue(state.designSystem, name)
-        if (value !== null) return value
+        if (value !== null) {
+          if (value.includes('var(')) {
+            seen.add(name)
+          }
+
+          return value
+        }
       }
 
       if (fallback) {