Is it possible to turn visible the invisible window's borders of Windows 10?

This may not be exactly what you are looking for, but it might assist you. If you go to System or right click "this computer" and go to properties, then on the left click "advanced system settings", click the button "settings" in the "performance" group box under the "advanced" tab, and then set the machine to "Adjust for best performance". After you do that click OK and let the settings kick in. Then try dragging a window around and you will see the "invisible" border box you are looking for that exists in prior versions of windows. Perhaps you can leverage that?

I have a conjecture: All custom themes are based on editing built-in aero.msstyles or aerolite.msstyles, located at %windir%\Resources\Themes\aero. aero comes with fixed 1px border, while aerolite has 1-8px border, configurable through registry or WSB (9px or above remains invisible). So far there is no public documentation on how to build msstyles from scratch. As such whether your theme has configurable visible border depends on which default msstyles was based on.

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