In the short-term you may find that restoring and re-maximising your browser window helps. Or try swapping around which monitor is considered by Windows to be your primary (#1) monitor. It sounds a bit weird that the screen real-estate from the other monitor is still available to the browser at all, so there may be a bug or misconfiguration in your operating system or graphics card drivers.
Despite all that, even when there are two valid screens connected and available, in at least some operating systems and some screen layouts, Firefox is still unable to position the Kee Network Authentication dialog correctly. It appears to work better in Chrome but I don’t have Edge available to test at the moment so can’t comment any further on the technical details apart from in Firefox.
It’s possible that some of the workarounds I mention below for Firefox will help for Edge on Windows 10 too but ultimately, Kee just asks the browser to display the dialog so in any cases where this does not happen correctly, there is a bug in that web browser. Kee never determines the position of these dialog windows, although implementing some code to do this may form at least some part of a workaround to improve the situation for at least some browsers in some screen configurations so I will explore that possibility in future. Unfortunately it will be a large job to test that any change to fix this problem you’re seeing will not create other equally annoying problems in one of the many other browsers and operating systems so I’m not sure exactly when I will be able to commit to looking into this topic again.
In Firefox, this behaviour is probably due to, or at least very related to, this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1271047
If that bug gets fixed, Kee may work as expected in Firefox without any further changes.
If it still ends up on the wrong screen then:
There is another Firefox bug that needs to be found and fixed AND
We should be able to change Kee so that it can work around that remaining bug by calculating and specifying the exact position required in order to position the dialog in the centre of the screen.
There is a workaround for the current situation which sounds too bad to consider adding to Kee - we’d have to make the window move around the screen after it loads for every user, regardless of multi-monitor status or the browser they are using. I don’t intend to implement that workaround.
If the Firefox bug never gets fixed, there is hope on the horizon from a very early draft for some new browser technology that will add support for multiple screens: https://github.com/webscreens/window-placement. This would require far more significant changes to Firefox but since it would be a shiny new feature that benefits web sites as well as browser extensions, they may choose to prioritise that over fixing the existing bug.