fix(overlay): show plain subtitle line immediately on tokenization cache miss (#184)

src/core/services/subtitle-processing-controller.ts

@@ -38,6 +38,9 @@ export function createSubtitleProcessingController(
       : DEFAULT_SUBTITLE_TOKENIZATION_CACHE_LIMIT;
   let latestText = '';
   let lastEmittedText = '';
+  // Tracks the latest provisional plain emit across rapid changes and loop retries
+  // so the same line is never shown plain twice.
+  let lastPlainEmittedText: string | null = null;
   let cacheGeneration = 0;
   let lastEmittedGeneration = 0;
   let processing = false;
@@ -81,9 +84,12 @@ export function createSubtitleProcessingController(
         const startedAtMs = now();
 
         if (!text.trim()) {
-          deps.emitSubtitle({ text, tokens: null });
+          if (lastPlainEmittedText !== text) {
+            deps.emitSubtitle({ text, tokens: null });
+          }
           lastEmittedText = text;
           lastEmittedGeneration = generation;
+          lastPlainEmittedText = null;
           break;
         }
 
@@ -93,11 +99,25 @@ export function createSubtitleProcessingController(
           if (cachedTokenized) {
             output = cachedTokenized;
           } else {
+            // Cache miss: show the plain line on time; the tokenized payload
+            // upgrades it once ready. Skipped on refreshes of an already
+            // emitted line so downstream consumers never see a downgrade.
+            if (text !== lastEmittedText && text !== lastPlainEmittedText) {
+              deps.emitSubtitle({ text, tokens: null });
+              lastPlainEmittedText = text;
+            }
             const tokenized = await deps.tokenizeSubtitle(text);
+            // A null result is a transient tokenizer failure, not a verdict on
+            // the line: caching the plain fallback would pin it untokenized for
+            // every later occurrence.
             if (tokenized) {
               output = tokenized;
+              // A result computed before an invalidation must not repopulate the
+              // fresh cache, or the retry below would serve the stale entry.
+              if (generation === cacheGeneration) {
+                setCachedTokenization(text, tokenized);
+              }
             }
-            setCachedTokenization(text, output);
           }
         } catch (error) {
           deps.logDebug?.(`Subtitle tokenization failed: ${(error as Error).message}`);
@@ -118,9 +138,16 @@ export function createSubtitleProcessingController(
           continue;
         }
 
-        deps.emitSubtitle(output);
+        // An untokenized result adds nothing when this line was already shown,
+        // either provisionally or as an earlier full emit (failed refresh) —
+        // emitting it would duplicate or downgrade what is on screen.
+        const plainAlreadyShown = lastPlainEmittedText === text || lastEmittedText === text;
+        if (!(output.tokens === null && output.text === text && plainAlreadyShown)) {
+          deps.emitSubtitle(output);
+        }
         lastEmittedText = text;
         lastEmittedGeneration = generation;
+        lastPlainEmittedText = null;
         deps.logDebug?.(
           `Subtitle tokenization delivered; elapsed=${now() - startedAtMs}ms, staleDrops=${staleDropCount}`,
         );
@@ -147,6 +174,14 @@ export function createSubtitleProcessingController(
         return;
       }
       latestText = text;
+      if (
+        processing &&
+        text !== lastPlainEmittedText &&
+        !tokenizationCache.has(normalizeSubtitleCacheKey(text))
+      ) {
+        deps.emitSubtitle({ text, tokens: null });
+        lastPlainEmittedText = text;
+      }
       processLatest();
     },
     refreshCurrentSubtitle: (textOverride?: string) => {
@@ -180,6 +215,7 @@ export function createSubtitleProcessingController(
       latestText = text;
       lastEmittedText = text;
       lastEmittedGeneration = cacheGeneration;
+      lastPlainEmittedText = null;
       return cached;
     },
     hasCachedSubtitle: (text: string) => {