This magic number constant should be replaced by a #define or const int

[liudr]

ArduinoCore-avr/cores/arduino/USBCore.cpp

Line 404 in 9f8d27f

if (!((_cmark + 1) & 0x3F))

Was looking at why I'm not able to emulate a keyboard with 8-byte bMaxPacketSize0. Took me a while to hunt this one down, was looking for 64 all over but found 0x3F.
Suggest:
#define bMaxPacketSize0_64 64
Then use (bMaxPacketSize0_64-1) for clarity.

I know it's just my opinion but this USB device stack could use more clarity and documentation. It would make it much easier to expand.