fix: prevent page unresponsive crashes from WebGL GPU stalls during session switch

Reduce terminal chunk size from 128KB to 32KB and use double-RAF between
chunks to give the WebGL renderer time to flush GPU operations. Also switch
cached buffer restore from direct terminal.write() to chunked writes —
the synchronous 256KB write was blocking the main thread for 5+ seconds
via synchronous ReadPixels calls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -1368,8 +1368,9 @@ class CodemanApp {
         this.terminal.write(chunk);
         offset += chunkSize;
 
-        // Schedule next chunk on next frame
-        requestAnimationFrame(writeChunk);
+        // Double-RAF: yields to both the browser's layout/paint AND the WebGL renderer's
+        // GPU flush. Single RAF can still block if ReadPixels is synchronous.
+        requestAnimationFrame(() => requestAnimationFrame(writeChunk));
       };
 
       // Start writing
@@ -3436,12 +3437,15 @@ class CodemanApp {
     // belt-and-suspenders guard for any edge cases.
     this._restoringFlushedState = true;
     try {
-      // Instant cache restore — show previous buffer immediately while network fetch runs
+      // Instant cache restore — show previous buffer via chunked write to avoid WebGL GPU stalls.
+      // Direct terminal.write() of large cached buffers (256KB+) can block the main thread
+      // for 5+ seconds while the WebGL renderer processes ReadPixels synchronously.
       const cachedBuffer = this.terminalBufferCache.get(sessionId);
       if (cachedBuffer) {
         this.terminal.clear();
         this.terminal.reset();
-        this.terminal.write(cachedBuffer);
+        await this.chunkedTerminalWrite(cachedBuffer);
+        if (selectGen !== this._selectGeneration) { this._restoringFlushedState = false; return; }
         this.terminal.scrollToBottom();
       }
 

src/web/public/constants.js

@@ -52,7 +52,7 @@ const TITLE_FLASH_INTERVAL_MS = 1500;       // Title flash rate
 const BROWSER_NOTIF_RATE_LIMIT_MS = 3000;   // Rate limit for browser notifications
 const AUTO_CLOSE_NOTIFICATION_MS = 8000;    // Auto-close browser notifications
 const THROTTLE_DELAY_MS = 100;              // General UI throttle delay
-const TERMINAL_CHUNK_SIZE = 128 * 1024;     // 128KB chunks for terminal data
+const TERMINAL_CHUNK_SIZE = 32 * 1024;      // 32KB chunks for terminal data (smaller to avoid WebGL GPU stalls)
 const TERMINAL_TAIL_SIZE = 256 * 1024;      // 256KB tail for initial load
 const SYNC_WAIT_TIMEOUT_MS = 50;            // Wait timeout for terminal sync
 const STATS_POLLING_INTERVAL_MS = 2000;     // System stats polling