Work around candidate parsing bug

Author: thecrypticace

packages/tailwindcss-language-service/src/util/getVariantsFromClassName.ts

@@ -20,8 +20,6 @@ export function getVariantsFromClassName(
     return { variants: [], offset: 0 }
   }
 
-  parts = parts.filter(Boolean)
-
   function isValidVariant(part: string) {
     if (allVariants.includes(part)) {
       return true
@@ -60,11 +58,36 @@ export function getVariantsFromClassName(
   let offset = 0
   let variants = new Set<string>()
 
-  for (let part of parts) {
+  for (let [index, part] of parts.entries()) {
+    // If we see an empty variant it's because:
+    //
+    // - The string contains consecutive top-level separators, e.g.
+    //   hover::flex
+    // - The string *ends* with a `:` which is a signal that the variant is done
+    //   and more should be suggested
+
+    // The first case isn't a valid class, partial or otherwise. The second one
+    // *is* valid because a user is probably in the middle of typing a utility.
+    //
+    // In both situations a `break` is sufficient to signal that the remainder
+    // of the string should be ignored when determining variants.
+    if (part === '') break
     if (!isValidVariant(part)) break
 
     variants.add(part)
-    offset += part.length + state.separator!.length
+
+    offset += part.length
+
+    // All variants must be succeeded by the separator (`:`) when used in a
+    // utility. However, Tailwind CSS <= v4.1.15 has a bug where we consider
+    // `bg-[` valid because we try to compile `bg-[:[color:red]` which in turn
+    // parses as a valid class when it obviously is not.
+    //
+    // To combat this we've done two things:
+    // - Add the offset to all variants *except* the last one
+    // - Allow an empty string in the last position to account for situations
+    //   where a utility name is currently being typed (e.g. `hover:`)
+    offset += index < parts.length - 1 ? state.separator!.length : 0
   }
 
   return { variants: Array.from(variants), offset }