Fusion
I'm running the latest version of Fusion 360 on a Mac, but I've witnessed this behavior in all past Mac versions since I started using Fusion 360 last Spring. When I use the Save as STL function and I turn on the "Send to 3D Print Utility" option and click OK, the foreground app switches from Fusion 360 to my specified app (Simplify3D), but it leaves the browser header and all of its contents on top of the Simplify3D screen, like this:
If I use Command-Tab to switch between running apps, this overlay remains on top of any app I switch to, until I switch back to Fusion 360. So, my workaround is this: after I export the STL to Simplify3D, I simply hit Command-Tab twice, and it disappears. This bug is not specific to any model, so it should be reproducible on any Mac. My guess is that the code that handles the switch-out (Command-Tab) is working properly, but the code that handles switching to the specified 3D print utility is skipping a step. I would call this a bug, and I hope to see it addressed in a future update. Thanks!
