An application should adapt itself to OS, not OS -> Application
This is only partially true. When it comes to UI guidelines, the application should respect the OS and adapt to it. If Windows says that a display area can be no smaller than the width of the titlebar (if present), then any window created with a titlebar should respect those rules. Since you created a window with a titlebar, mIRC needs to respect the rules. Since -f specifies a window attribute which is directly related to window size, it must respect all window size UI guidelines. This means as a scripter or *ANY* for windows development, you should never expect a window with a titlebar to be smaller than the titlebar width. Although mIRC *can* work around this issue in the way you described, it should not be the expected behaviour given what was said above.
