Opening Serial Monitor disconnects Port

[liderbug]

Arduino IDE 1.8.5 ARM, RasPi-3
Problem is with a FTD1232 USB<->TTL
esp8266-01 ( *** problem does not happen with a NodeMCU **)
Plug everything in, power up
Open Serial Monitor
Tools, Port, ttyUSB0
Serial Monitor goes away
Open Serial Monitor
Check port - blank - select, now USB1 is avail., select SM goes away
Upload
running, open SM - nothing - Serial.print no output
and on a terminal running "watch -n 1 'ls -l /dev/ttyUSB' " - Upload again
shows USB0/1 there, not there, now 0, gone, now 0, gone, now 1, now 0 while uploading

Seems I'm not the only one:
https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=205119&p=1271796#p1271796

[facchinm]

The tty name changes because it gets re-enumerated by the kernel when another program is already "owning" it (it can be the serial monitor or the uploader).
The problem here is about the port disappearance; it could be related with the exact FTDI model or drivers in use, since the IDE asserts DTR at serial monitor open (to trigger a reboot on boards like UNO/Nano etc) and this could have some effect on your FTDI addapter (namely, it could restart if its DTR line is connected to some other rail). Selecting the "generic" module should disable this behaviour (see https://github.com/esp8266/Arduino/blob/master/boards.txt#L25)