@All
There some "real-amiga" question for developers working with amiga screens and windowses.
Let imagine we grab a screenmode which is 1920x1080x32. Then, we use this:
Myscreen = OpenScreenTags(  NULL,
                                    SA_Width,     screen_width,
                                    SA_Height,    screen_height*2,
                                    SA_DisplayID, display_mode_id,
                                    SA_Depth,     8,
                                    SA_ShowTitle, FALSE,
                                    SA_Draggable, FALSE,
                                    TAG_DONE    );
So we open "double screen" as do back in times some games/apps to have double-buffering or whatever reassons. 
So while the real screenmode is for 1920x1080, we open a screen with 1920x2160. Then, we open the same window 1920x2160 on, to render things and have double-buffering as it was done in old days.
Now, problem is:
I need to open another window on top of it , which will overwrite  first half part of content and showups on the same place where old "double-buffered" window is placed. 
If we open a window on this screen of original size, like:
Window = OpenWindowTags(NULL,
     WA_CustomScreen, (ULONG)Myscreen,
     WA_IDCMP, IDCMP_RAWKEY,
     WA_Height, hardware_acceleration? Myscreen->Height/2 : Myscreen->Height,
     WA_StayTop,TRUE, WA_Activate,TRUE,
     WA_Borderless,TRUE, WA_RMBTrap,TRUE,
 TAG_DONE);
I.e. 1920x1080, then, we do have 2 windowses on the same screen, one original 1920x2160, and another one 1920x1080, and this second one is not overwrite first one, but instead shows ups in other window, which i can see only if i scrolldown workbench screen.  
Like, the second window draws not at 0,0 of the screen, but on on the second part of 2160 size.
Question is : if we have 1920x2160 screen, how to open a window of 1920x1080 size on the same screen, but which will overwrite at top and be visibly first one.