Allow one sketch to be uploaded to, and serial-monitored on, multiple ports

[StephenJD]

Is your feature request related to a problem? Please describe.
I am testing a sketch on multiple boards (various different Arduino flavours). If I try to open the IDE twice for the same sketch, it simply reverts to the original. To persuade the IDE to give me multiple windows I have to have a differently named sketch for each board. (I cannot find a way to launch multiple IDE's from Explorer.)
This is tedious, because any changes I make to the "master" sketch, I have to copy and paste to all the others. It also means I have multiple sketches on disk, only one of which I actually need.

Describe the solution you'd like
The nicest solution would be to enable one IDE to upload to, and monitor, multiple ports, instead of just one. If this is not possible, then when "reopening" a sketch that an IDE is already showing, have it ask if you want to allow multiple IDE's for this sketch. As a last resort, allow windows explorer to launch multiple instances of the IDE.

Describe alternatives you've considered
Use Arduino 1.8 which allows windows explorer to launch multiple IDE's with the same sketch.

Additional context

Additional requests:

• Allow one sketch to be uploaded to, and serial-monitored on, multiple ports #583 (comment)
• Suggestion: Serial monitor having option to select the port independently. Arduino#10920
• Need separate settings for Serial Monitor port and Upload Port Arduino#8908
• Separate devices for uploading and for the serial console Arduino#7247
• https://forum.arduino.cc/t/serial-monitor-port/1060383

[per1234]

The IDE now does have the capability to open the same sketch in multiple windows via File > New Window.

It can be awkward at times though because the most recently opened sketch is loaded (equivalent to the behavior when starting the IDE), which is not necessarily the sketch that was open in the window you accessed File > New Window from.

So I'll leave this open for now in case the UX experts want to take a look. I suspect that File > New Window was introduced as a side effect of updating to a new version of the Eclipse Theia IDE framework.

[per1234]

The IDE now does have the capability to open the same sketch in multiple windows via File > New Window.

The feature stopped working at 2.0.0-rc7 and was later removed via #1071