How to cancel a stuck file upload?

I’m having occasional upload issues with the UI. Sometimes when I try to upload artwork for printing, the Glowforge app will seem to freeze. The file never uploads, and if I try to upload it again, I get this dialogue box:

Problem is, that upload NEVER completes, and after waiting anywhere from 15 to 30 minutes with no change, I have to completely close out of the browser (sometimes losing my design progress) to clear the upload and try again. This afternoon I have been trying to upload my file (which I know is compatible with the GF app, as it’s a design element I’ve printed before) for two straight hours, and I still have nothing to print.

I wondered if this might be an issue with internet speed – sometimes my service isn’t super great – but even with a slower-than-average connection, it still shouldn’t take half an hour to upload a 1.4MB file! (I mean, I’m not on dialup.)

Is there any way in the app to manually cancel an upload, or am I stuck just doing what I’m doing and starting over when this happens?

I haven’t seen that in years, but early on in development we sometimes saw this and @takitus discovered that if you open another tab in the browser and load a simple file that processes quickly, like a circle, and let it process all the way till the button flashes - then cancel that op and return to the original tab it would be cleared.
As I said, I haven’t seen that for years, so I suspect another issue, but you could try that.