Irv, I'm sorry you are having problems and I think it is very reasonable to bump the priority of the issue as you are not alone, even though, as Terry notes, a complete resolution may require changes elsewhere. I am looking more into this now. macOS 10.15 Catalina has introduced a number of changes in the interest of improving your security and some of these have had an impact on both Python and on Tcl/Tk which IDLE uses to power its GUI. While most of the 10.15-specific issues are addressed in the latest release of Python (3.8.3), the issue of double-clicking a file while IDLE is already open is not yet. However, I notice you mention that you are using IDLE from Python 3.7.3. If so, be aware that that is not the latest release of either Python 3 or Python 3.7 (3.7.3 predates Catalina.) 3.7.7 is the most recent for 3.7 but Python 3.8 is now current and its most recent release, 3.8.3, has the most up-to-date support for 10.15. If possible, you should update to 3.8.3 or at least to 3.7.7, which has some. That won't fix everyting but it may make some issues go away.

You also mention the apparent closing of an IDLE edit window when you open another edit window. I have not seen nor am aware of reports of behavior exactly like that; however, I think you might be running into a similar situation to what Andrew reports in an earlier comment (msg366978) in this issue. What you may be going on is a case where trying to open another file to edit in IDLE unexpectedly causes a second instance of IDLE to open, with the original file still open in the original instance (but perhaps with its windows now obscured by other windows) and with the second file open in an edit window in the additional IDLE instance.  That can be very confusing as having more than one instance of the same app running is normally not supposed to happen. One way to tell that this is happening is to check the macOS Dock and see if there is more than one IDLE icon present with the active indicator. If you switch to the other IDLE instance, your original edit window should still be present. (You could also use Mission Control features to see all open windows.)  Let us know if you can determine that the closing window is the second IDLE case with 3.7.3; this problem may not occur with IDLE from 3.8.3 but let us know.

By the way, I am assuming here that you are using Python 3.7.3 downloaded from the python.org website. Let us know if that is not the case.