Arduino 1.8.19, UNO R4 WIFI, Dot Star featherwing (6x12 APA102 array) connected to SPI, pin13 (SCK) and pin11 (MOSI). Featherwing power is from a separate supply and there is a ground connection from Featherwing to the UNO.
So when the upload runs, SCK is active and MOSI is constant high (at least when SCK is running). This causes the Featherwing to sequentially light up its LEDs full white at a rate that seems to match the rate of the "=" signs on the IDE display. After that my program controls the Featherwing perfectly. (At least on one simple script. On another more complicated script it seems to also crash the PORT connection and the script does not run correctly. But that's not the immediate problem)
I am on 1.8.19 because I am hosting on an RPi-5 and I could not get that "unsupported" up-to-date version (if you know what I mean) to install on my RPi-5.
Any way, why any activity on SPI pins at all during upload???