Hi,
Have you identified this? Fieldset control is not working after the recent MAC OS, Sierra + Safari v11 update. You can check this in your 2.5 example site. Open this page( https://examples2.ext.net/#/Form/Groups/Basic/) in Safari (v11) browser and try to expand/collapse fieldsets to replicate the issue.
We use Ext.Net 2.5 version on our website. We will be thankful to you if you provide a quick fix for this issue.