Serial Monitor/Plotter don't use per-window port selection

[AlbyIanna]

When you open Serial Monitor or Serial Plotter in multiple IDE windows, they all use the same port, even though you have a different serial port selected from the Board Selector or Tools > Port menu in each window.

[per1234]

Hi @AlbyIanna. Can you confirm that, in regards to Serial Monitor, this is a regression that will only be introduced once #597 is merged (i.e., it does not occur when using any version of the IDE from the main branch at this time)?

[AlbyIanna]

@per1234 Yes, I confirm it's a bug introduced in the branch serial-plotter. We decided to consider this issue an edge case, so we're not planning to solve this before the release. That absolutely does not mean that it's not an important issue, it's just that we have more urgent stuff to do at the moment.

[per1234]

Even though this issue is believed to be a new regression, just to create a linkage to related information, I'll add that something similar was mentioned incidentally in another issue:
#584 (comment)

If a second IDE is open on another port, the one that looses the connection reconnects to the second IDE's port.

as well as this:
#271 (comment)

When you click the Serial Monitor icon in window 2, it attempts to open serial port 1 rather than the port it just uploaded to! You have to explicitly pick the second port AFTER you upload to open both ports. That's confusing. Whatever board and port you select for a given window should be the default for all the serial operations associated with that window.

[per1234]

Something that might not be clear from this report is that the Serial Monitors show the merged output from the ports selected in each window. There is a demonstration of that here: #661 (comment)