Fix bug where top section of the previous preview content appearing

when the preview window is re-enabled and the current preview process is
taking more than 500ms and previewDelayed is triggered

  fzf --preview 'sleep 1; date; seq 1000' --bind space:toggle-preview

src/terminal.go

@@ -3100,6 +3100,10 @@ func (t *Terminal) Loop() {
 							t.previewBox.Set(reqPreviewEnqueue,
 								previewRequest{t.previewOpts.command, t.pwindow, t.evaluateScrollOffset(), list})
 						}
+					} else {
+						// Discard the preview content so that it won't accidentally appear
+						// when preview window is re-enabled and previewDelay is triggered
+						t.previewer.lines = nil
 					}
 				}
 			case actTogglePreviewWrap: