But this "limitation" can be removed by creating custom MDI @picwin Windows without dealing with standard Windows API
The limitation would not be removed-- it would just be bypassed by treating the inner canvas "Static" control that you found as an inner part of the display area. The window itself (AND the display area) would still be limited by the OS, mIRC would just be able to draw to a smaller part of the display area. This would be a
workaround that would lead to tons of inconsistencies. For instance, if mIRC resized the inner canvas to be smaller than the display area, $window(@mywindow).w/h would *still* return the entire display area, but you would not be able to draw to all of it because the canvas would be clipped. This would probably confuse scripters trying to incorrectly manipulate small windows. Most scripts I know base their drawing calculations on $window(@mywindow).w/h, so this would lead to problems.
(so, create custom border, etc..).
You can already create custom borders. The onus is on you to do it, otherwise you should expect default Windows behaviour. Again, see /help /window and make use of the switches available to you.
lots of programs prefers have a custom window interface
Custom window interfaces (at least the ones I know) do not use bordered windows. They create borderless windows and code their own titlebar logic-- you should do this too.
And I don't read in mIRC.chm that we must know "Windows API" before learning mIRC's Scripting.
This is exactly the point. You're the one coming in with assumptions about the Windows API-- you're the one who brought up API calls and implementation details about the window class. The point is this:
API aside, to a layman, "display area" means the window size minus the border. This is what the help file talks about. For example, if you make a window with a *display area* that is 100x100 and then resize it to 200x200, you have a *display area* of 200x200, this goes for resizing downwards as well. If we can agree on that, then there is no issue here, because if the display area is just the non-border part of the window, it can never be smaller than the 114px you reported when a titlebar is present. If we count "display area" as the entire white background in that screenshot you posted, it is directly tied to the window size. However if we count "display area" as a subsection of the actual display area, we have to introduce concepts like "canvas size" which can be arbitrarily large as compared to the actual window and it gets confusing fast.
