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/InputHandler.ts

@@ -928,7 +928,6 @@ export class InputHandler implements IInputHandler {
         case 47: // alt screen buffer
         case 1047: // alt screen buffer
           this._terminal.buffers.activateAltBuffer();
-          this._terminal.selectionManager.setBuffer(this._terminal.buffer);
           this._terminal.viewport.syncScrollArea();
           this._terminal.showCursor();
           break;
@@ -1100,7 +1099,6 @@ export class InputHandler implements IInputHandler {
           // if (params[0] === 1049) {
           //   this.restoreCursor(params);
           // }
-          this._terminal.selectionManager.setBuffer(this._terminal.buffer);
           this._terminal.refresh(0, this._terminal.rows - 1);
           this._terminal.viewport.syncScrollArea();
           this._terminal.showCursor();