Do these errors get reported to Xojo after clicking Send?
No. They get reported to you. See Session.JavascriptError.
@Greg O’Lone I don’t have any of my own JavaScript in this project so it must be part of the Web framework. How do I work around it? How do I help you to debug it? There is no pattern to when it occurs and it happens less often than not but too frequently.
Look for code where you’re setting the top of a control to 180 in an open event. That’s the most likely issue. Otherwise, getting a description of what the user was doing (or what was on screen at that moment) may be helpful. When I run into issues like this, I tend to use Rewind until I actually catch it on tape 
If you see it again, note the Id of the control (here PEY2v9t8), open the browser developer tools, and do a search for that element. I love chrome, because it outlines the selected element.
Then you will know which control is triggering the error.
From what I understand, it seems JavaScript is trying to set the top of a control that is no longer here, or not yet fully formed. Maybe a WebContainer you just closed… The top at 180 px could probably help figure out which control it is as well.
Thank you both! It’s likely a dynamically embedded WebContainer with a calculated Top value. Open events are not used so it may be a Resized event occurring between instantiations. I’ll use Rewind in an attempt to capture it and also note the control id to determine which WebContainer.