We noticed that the content of Task Pane has been limited for a specific height in Office Online, Safari browser only. Firstly, we noticed it on our custom add-in, but the issue is reproducible for Script Lab too (for details, see a video https://drive.google.com/file/d/1AZrBpt7VN-sV1O9r9AJAHnYmt5EOPrwY/view?usp=sharing). Do you happen to know, why did the issue appear? We worked with Task Pane in safari a few weeks ago, and it works fine, without issues.
This issue should now be fixed (please see https://github.com/OfficeDev/office-js/issues/1444).