Fix O(n²) payload accumulation in DCS/OSC/APC handlers

Replace repeated string concatenation with a chunk buffer that joins
once on completion. Fixes #5916.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/common/parser/ApcParser.ts

@@ -7,6 +7,7 @@ import { IApcHandler, IHandlerCollection, ApcFallbackHandlerType, IApcParser, IS
 import { ParserConstants } from 'common/parser/Constants';
 import { utf32ToString } from 'common/input/TextDecoder';
 import { IDisposable } from 'common/Types';
+import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
 
 const EMPTY_HANDLERS: IApcHandler[] = [];
 
@@ -153,13 +154,13 @@ export class ApcParser implements IApcParser {
 export class ApcHandler implements IApcHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = '';
+  private _data = new PayloadStringBuffer();
   private _hitLimit: boolean = false;
 
   constructor(private _handler: (data: string) => boolean | Promise<boolean>) { }
 
   public start(): void {
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
   }
 
@@ -167,9 +168,7 @@ export class ApcHandler implements IApcHandler {
     if (this._hitLimit) {
       return;
     }
-    this._data += utf32ToString(data, start, end);
-    if (this._data.length > ApcHandler._payloadLimit) {
-      this._data = '';
+    if (this._data.add(utf32ToString(data, start, end), ApcHandler._payloadLimit)) {
       this._hitLimit = true;
     }
   }
@@ -179,18 +178,18 @@ export class ApcHandler implements IApcHandler {
     if (this._hitLimit) {
       ret = false;
     } else if (success) {
-      ret = this._handler(this._data);
+      ret = this._handler(this._data.toString());
       if (ret instanceof Promise) {
         // need to hold data until `ret` got resolved
         // dont care for errors, data will be freed anyway on next start
         return ret.then(res => {
-          this._data = '';
+          this._data.reset();
           this._hitLimit = false;
           return res;
         });
       }
     }
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
     return ret;
   }

src/common/parser/DcsParser.ts

@@ -8,6 +8,7 @@ import { IDcsHandler, IParams, IHandlerCollection, IDcsParser, DcsFallbackHandle
 import { utf32ToString } from 'common/input/TextDecoder';
 import { Params } from 'common/parser/Params';
 import { ParserConstants } from 'common/parser/Constants';
+import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
 
 const EMPTY_HANDLERS: IDcsHandler[] = [];
 
@@ -140,7 +141,7 @@ EMPTY_PARAMS.addParam(0);
 export class DcsHandler implements IDcsHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = '';
+  private _data = new PayloadStringBuffer();
   private _params: IParams = EMPTY_PARAMS;
   private _hitLimit: boolean = false;
 
@@ -152,7 +153,7 @@ export class DcsHandler implements IDcsHandler {
     // perf optimization:
     // clone only, if we have non empty params, otherwise stick with default
     this._params = (params.length > 1 || params.params[0]) ? params.clone() : EMPTY_PARAMS;
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
   }
 
@@ -160,9 +161,7 @@ export class DcsHandler implements IDcsHandler {
     if (this._hitLimit) {
       return;
     }
-    this._data += utf32ToString(data, start, end);
-    if (this._data.length > DcsHandler._payloadLimit) {
-      this._data = '';
+    if (this._data.add(utf32ToString(data, start, end), DcsHandler._payloadLimit)) {
       this._hitLimit = true;
     }
   }
@@ -172,20 +171,20 @@ export class DcsHandler implements IDcsHandler {
     if (this._hitLimit) {
       ret = false;
     } else if (success) {
-      ret = this._handler(this._data, this._params);
+      ret = this._handler(this._data.toString(), this._params);
       if (ret instanceof Promise) {
         // need to hold data and params until `ret` got resolved
         // dont care for errors, data will be freed anyway on next start
         return ret.then(res => {
           this._params = EMPTY_PARAMS;
-          this._data = '';
+          this._data.reset();
           this._hitLimit = false;
           return res;
         });
       }
     }
     this._params = EMPTY_PARAMS;
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
     return ret;
   }

src/common/parser/OscParser.ts

@@ -7,6 +7,7 @@ import { IOscHandler, IHandlerCollection, OscFallbackHandlerType, IOscParser, IS
 import { OscState, ParserConstants } from 'common/parser/Constants';
 import { utf32ToString } from 'common/input/TextDecoder';
 import { IDisposable } from 'common/Types';
+import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
 
 const EMPTY_HANDLERS: IOscHandler[] = [];
 
@@ -194,13 +195,13 @@ export class OscParser implements IOscParser {
 export class OscHandler implements IOscHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = '';
+  private _data = new PayloadStringBuffer();
   private _hitLimit: boolean = false;
 
   constructor(private _handler: (data: string) => boolean | Promise<boolean>) { }
 
   public start(): void {
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
   }
 
@@ -208,9 +209,7 @@ export class OscHandler implements IOscHandler {
     if (this._hitLimit) {
       return;
     }
-    this._data += utf32ToString(data, start, end);
-    if (this._data.length > OscHandler._payloadLimit) {
-      this._data = '';
+    if (this._data.add(utf32ToString(data, start, end), OscHandler._payloadLimit)) {
       this._hitLimit = true;
     }
   }
@@ -220,18 +219,18 @@ export class OscHandler implements IOscHandler {
     if (this._hitLimit) {
       ret = false;
     } else if (success) {
-      ret = this._handler(this._data);
+      ret = this._handler(this._data.toString());
       if (ret instanceof Promise) {
         // need to hold data until `ret` got resolved
         // dont care for errors, data will be freed anyway on next start
         return ret.then(res => {
-          this._data = '';
+          this._data.reset();
           this._hitLimit = false;
           return res;
         });
       }
     }
-    this._data = '';
+    this._data.reset();
     this._hitLimit = false;
     return ret;
   }

src/common/parser/PayloadStringBuffer.ts

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) 2026 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+/**
+ * Accumulates string payloads from multiple chunks without O(n²) string concatenation.
+ */
+export class PayloadStringBuffer {
+  private _chunks: string[] = [];
+  private _length = 0;
+
+  public reset(): void {
+    this._chunks.length = 0;
+    this._length = 0;
+  }
+
+  /**
+   * @returns true if the payload limit was exceeded (buffer is cleared in that case)
+   */
+  public add(chunk: string, limit: number): boolean {
+    this._chunks.push(chunk);
+    this._length += chunk.length;
+    if (this._length > limit) {
+      this.reset();
+      return true;
+    }
+    return false;
+  }
+
+  public toString(): string {
+    return this._chunks.join('');
+  }
+}