element
With the dis-assembled dryer for test purposes, you arent getting much, if any, airflow over the element, so it can get real hot real quick. When fully assembled with the drum installed, even if you have the door open for pictures while it's running, the blower will pull a decent amount of air over the coils, properly pulling the heat into the drum instead of letting it build up and get cherry red glowing. Then, with the door closed, you've got approx 100% of the airflow pulling air over the element. That way it may glow a bit, but not much, and the correct overall airflow from intake over the element, into the drum, thru the clothes, out the back right perf area, down to the blower and out of the cabinet will all work as designed.
It's wise to test elements and such while re-assembling, but not for long. The airflow and safety issues are wrong for more than a quick test.