We faced the following issue when previewing a big enough report. Here are the steps to reproduce the bug:

1) after the report gets loaded, press the Preview button from the Parameters area.

2) as soon as the report starts loading, the class="trv-loading" is added to the Parameters area. While still report loading is in progress, press the "Stop Rendering" button.

3) When the message Report processing was canceled. is displayed, the Parameters area remains inactive. This happens because its class="trv-loading" is not removed as it is expected.