Fix issue with resizing terminal.

Need to ensure the active buffer is always being used.
Fixes https://github.com/xtermjs/xterm.js/issues/824 .

src/SelectionManager.ts

@@ -98,7 +98,6 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
 
   constructor(
     private _terminal: ITerminal,
-    private _buffer: IBuffer,
     private _charMeasure: CharMeasure
   ) {
     super();
@@ -109,6 +108,10 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
     this._activeSelectionMode = SelectionMode.NORMAL;
   }
 
+  private get _buffer(): IBuffer {
+      return this._terminal.buffers.active;
+  }
+
   /**
    * Initializes listener variables.
    */
@@ -139,16 +142,6 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
     this._enabled = true;
   }
 
-  /**
-   * Sets the active buffer, this should be called when the alt buffer is
-   * switched in or out.
-   * @param buffer The active buffer.
-   */
-  public setBuffer(buffer: IBuffer): void {
-    this._buffer = buffer;
-    this.clearSelection();
-  }
-
   public get selectionStart(): [number, number] { return this._model.finalSelectionStart; }
   public get selectionEnd(): [number, number] { return this._model.finalSelectionEnd; }