SAMD21 USBDeviceClass::ISRHandler() called continuously.

Running the MIDIUSB library on MKR1000, I discovered with a debug statement in USBDeviceClass::ISRHandler() within USBCore.cpp, the handler is called continuously.

Specifically, adding Serial1.println("Handler Called"); to the first line of ISRHandler() exposed the problem.

I did not look deeper to find the specific flag in DEVICE.INTFLAG causing the problem, and whether or not that flag is required for enumeration. However, executing this statement after enumeration seems to solve the problem.
USB->DEVICE.INTENCLR.reg = 0xFF;

Start in USBCore.cpp at USBDeviceClass::ISRHandler() and reproduce the behavior with the MIDIUSB library on an MKR1000. Inspect DEVICE.INTFLAG and the effect of USB->DEVICE.INTENCLR.reg = 0xFF; done means the handler no longer runs continuously after enumeration without breaking USB enumeration.

Written by the indexing model from the issue text.