fix test and improve name

src/browser/TestUtils.test.ts

@@ -254,6 +254,9 @@ export class MockBuffer implements IBuffer {
   public getWhitespaceCell(attr?: IAttributeData): ICellData {
     throw new Error('Method not implemented.');
   }
+  public clearMarkers(): void {
+    throw new Error('Method not implemented.');
+  }
 }
 
 export class MockRenderer implements IRenderer {

src/common/buffer/Buffer.ts

@@ -16,7 +16,6 @@ import { DEFAULT_CHARSET } from 'common/data/Charsets';
 import { ExtendedAttrs } from 'common/buffer/AttributeData';
 
 export const MAX_BUFFER_SIZE = 4294967295; // 2^32 - 1
-const enum BufferState { CLEARING = 'clearing' }
 
 /**
  * This class represents a terminal buffer (an internal state of the terminal), where the
@@ -44,7 +43,7 @@ export class Buffer implements IBuffer {
   private _whitespaceCell: ICellData = CellData.fromCharData([0, WHITESPACE_CELL_CHAR, WHITESPACE_CELL_WIDTH, WHITESPACE_CELL_CODE]);
   private _cols: number;
   private _rows: number;
-  private _state: string | undefined;
+  private _isClearing: boolean = false;
 
   constructor(
     private _hasScrollback: boolean,
@@ -587,12 +586,12 @@ export class Buffer implements IBuffer {
   }
 
   public clearMarkers(): void {
-    this._state = BufferState.CLEARING;
+    this._isClearing = true;
     for (const marker of this.markers) {
       marker.dispose();
     }
     this.markers = [];
-    this._state = undefined;
+    this._isClearing = false;
   }
 
   public addMarker(y: number): Marker {
@@ -626,7 +625,7 @@ export class Buffer implements IBuffer {
   }
 
   private _removeMarker(marker: Marker): void {
-    if (this._state !== BufferState.CLEARING) {
+    if (this._isClearing !== BufferState.CLEARING) {
       this.markers.splice(this.markers.indexOf(marker), 1);
     }
   }