For this Renoise must verify what image resolution you have on each monitor. Thus it is possible to establish X-Y coordinates, in order to determine a reference point to save (for example, upper left corner of the window, say X = 432, and Y = 240 (it would actually be a 2-value table), on a 1920x1080 monitor). This point within the surface must be a property of each tool window instance and 2 data should be saved each time you close a tool window, the XY coordinates (the table) of the point relative to the monitor surface, and an index of monitor with its resolution (XY, also another table), to be able to make checks. All this implemented in the API. Of course, all these values should be read and write and observable, in addition to being able to place it in the foreground, so that the programmer can establish where he wants the window to appear and always be able to check it. This would allow to make a preference framework so that the same user can set these values as he wishes.
All this mentioned before does not exist. In addition, currently Renoise 3.2.1 still has problems with the size of its windows and position, at least in Windows 10 (where it appears and if it appears maximized or in window). It seems that it does not correctly record the position and / or size of certain windows. This data is saved in the Renoise preferences.xml file each time the user logs off. I don’t understand why it fails. It seems that simply, whenever you need this data, you should check if the used image monitors remain the same.
In addition, you will have problems with different resolution image monitors and scaling. This generates a somewhat absurd situation. If you have a 2K or 4K monitor and another full HD monitor, and you increase the scaling, for example to 200%, you will have to fuck yourself and use 200% on your full HD monitor, when this makes no sense. For now, Renoise cannot deal correctly with monitors of different resolutions, so I, for the moment, only recommend using monitors of the same or similar resolution. Any other configuration will force any of the windows to be fuzzy, which is anti-productive.
Hopefully @taktik read all this and decide to improve it, although he is probably very aware of all this. In addition, it would be reasonable for at least the tools to have their own scaling. Having to duplicate the icons so that everything looks good costs more than even programming (at least in large and elaborate window tools). And unfortunately, the API has no control of the text font size or flipping. At least it would alibiar the use of monitors with different resolutions and the use of window tools. It is quite common that today any user who owns a home studio has a new monitor of larger size and resolution and an old one of smaller size and resolution.
We can also talk about what happens with the window plugins VSTi and their scaling, beyond saving their position for the image monitor.