What is non-client mouse event? And how can I determine on which object mouse has been clicked?

Overview

Mouse non-client area are event names starts with WM_NC (Window Message Non Client). These messages are in same format as mouse client area events. The only difference is these are posted to the window when mouse is interacted in non-client area.

• WM_NCHITTEST - Mouse non-client area test
• WM_NCLBUTTONDBLCLK - Mouse non-client area left button double click
• WM_NCLBUTTONDOWN - Mouse non-client area left button down
• WM_NCLBUTTONUP - Mouse non-client area left button up
• WM_NCMBUTTONDBLCLK - Mouse non-client area middle button double click
• WM_NCMBUTTONDOWN - Mouse non-client area middle button down
• WM_NCMBUTTONUP - Mouse non-client area middle button up
• WM_NCMOUSEMOVE - Mouse non-client area x or y movement
• WM_NCRBUTTONDBLCLK - Mouse non-client area right button double click
• WM_NCRBUTTONDOWN - Mouse non-client area right button down
• WM_NCRBUTTONUP - Mouse non-client area right button up
• WM_NCXBUTTONDBLCLK - Mouse non-client area X button up
• WM_NCXBUTTONDOWN - Mouse non-client area X button up
• WM_NCXBUTTONUP - Mouse non-client area X button up

WM_NCHITTEST

Mouse position can be obtained from lparam

xPos = GET_X_LPARAM(lParam); 
yPos = GET_Y_LPARAM(lParam);

WM_NCHITTEST event parameters can be passed to DefWindowProc() which will return a code to know which object is clicked. Here are the return codes from DefWindowProc().

Return code/value	Description
HTBORDER 18	In the border of a window that does not have a sizing border.
HTBOTTOM 15	In the lower-horizontal border of a resizable window (the user can click the mouse to resize the window vertically).
HTBOTTOMLEFT 16	In the lower-left corner of a border of a resizable window (the user can click the mouse to resize the window diagonally).
HTBOTTOMRIGHT 17	In the lower-right corner of a border of a resizable window (the user can click the mouse to resize the window diagonally).
HTCAPTION 2	In a title bar.
HTCLIENT 1	In a client area.
HTCLOSE 20	In a Close button.
HTERROR -2	On the screen background or on a dividing line between windows (same as HTNOWHERE, except that the DefWindowProc function produces a system beep to indicate an error).
HTGROWBOX 4	In a size box (same as HTSIZE).
HTHELP 21	In a Help button.
HTHSCROLL 6	In a horizontal scroll bar.
HTLEFT 10	In the left border of a resizable window (the user can click the mouse to resize the window horizontally).
HTMENU 5	In a menu.
HTMAXBUTTON 9	In a Maximize button.
HTMINBUTTON 8	In a Minimize button.
HTNOWHERE 0	On the screen background or on a dividing line between windows.
HTREDUCE 8	In a Minimize button.
HTRIGHT 11	In the right border of a resizable window (the user can click the mouse to resize the window horizontally).
HTSIZE 4	In a size box (same as HTGROWBOX).
HTSYSMENU 3	In a window menu or in a Close button in a child window.
HTTOP 12	In the upper-horizontal border of a window.
HTTOPLEFT 13	In the upper-left corner of a window border.
HTTOPRIGHT 14	In the upper-right corner of a window border.
HTTRANSPARENT -1	In a window currently covered by another window in the same thread (the message will be sent to underlying windows in the same thread until one of them returns a code that is not HTTRANSPARENT).
HTVSCROLL 7	In the vertical scroll bar.
HTZOOM 9	In a Maximize button.

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