Puzzle about the fourth parameter of "ScrollWindow" API function in Windows Programming - c

Question

My code is shown below. When the program is running, I click the right mouse button first, then I click the left mouse button. The result is shown as in the first picture. According to the help document of ScrollWindow function, if the fourth parameter is NULL, the entire client area should be scrolled. Why is there a 10 pixels gap at x = 30 device units?
I'm wondering why the result is not shown like the second picture.
#include <windows.h>
LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT ("HelloWin") ;
HWND hwnd ;
MSG msg ;
WNDCLASS wndclass ;
wndclass.style = CS_HREDRAW | CS_VREDRAW ;
wndclass.lpfnWndProc = WndProc ;
wndclass.cbClsExtra = 0 ;
wndclass.cbWndExtra = 0 ;
wndclass.hInstance = hInstance ;
wndclass.hIcon = LoadIcon (NULL, IDI_APPLICATION) ;
wndclass.hCursor = LoadCursor (NULL, IDC_ARROW) ;
wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
wndclass.lpszMenuName = NULL ;
wndclass.lpszClassName = szAppName ;
if (!RegisterClass (&wndclass))
{
MessageBox (NULL, TEXT ("This program requires Windows NT!"),
szAppName, MB_ICONERROR) ;
return 0 ;
}
hwnd = CreateWindow (szAppName, // window class name
TEXT ("The Hello Program"), // window caption
WS_OVERLAPPEDWINDOW, // window style
CW_USEDEFAULT, // initial x position
CW_USEDEFAULT, // initial y position
CW_USEDEFAULT, // initial x size
CW_USEDEFAULT, // initial y size
NULL, // parent window handle
NULL, // window menu handle
hInstance, // program instance handle
NULL) ; // creation parameters
ShowWindow (hwnd, iCmdShow) ;
UpdateWindow (hwnd) ;
while (GetMessage (&msg, NULL, 0, 0))
{
TranslateMessage (&msg) ;
DispatchMessage (&msg) ;
}
return msg.wParam ;
}
LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc ;
PAINTSTRUCT ps ;
RECT rect;
switch (message)
{
case WM_PAINT:
hdc = BeginPaint (hwnd, &ps) ;
EndPaint (hwnd, &ps) ;
return 0 ;
case WM_LBUTTONDOWN:
hdc = GetDC(hwnd);
SetRect(&rect, 30, 0, 70, 100);
ScrollWindow(hwnd, 10, 0, NULL, &rect);
UpdateWindow(hwnd);
ReleaseDC(hwnd, hdc);
return 0;
case WM_RBUTTONDOWN:
hdc = GetDC(hwnd);
Ellipse(hdc, 0, 0, 100, 100);
ReleaseDC(hwnd, hdc);
return 0;
case WM_DESTROY:
PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, message, wParam, lParam) ;
}

Answer 1

if the fourth parameter is NULL, the entire client area should be scrolled.
You also specify a clipping rectangle (the 5th parameter), so of course not the entire client area will be scrolled. But actually that's not relevant for the problem.
Why is there a 10 pixels gap at x = 30 device units?
Because you don't paint that gap when Windows tells you to do so.
From MSDN:
The area uncovered by ScrollWindow is not repainted, but it is combined into the window's update region. The application eventually receives a WM_PAINT message notifying it that the region must be repainted.
Your WM_PAINT handler does ... nothing except lying to Windows by validating the update region.
Fix your code by doing all the painting only in WM_PAINT. When you scroll in WM_LBUTTONDOWN you will also have to increment a variable that stores the scroll position. Add the scroll position to the coordinates you pass to Ellipse() in WM_PAINT. Now you should get a result like the 2nd picture.
I suggest to find a good tutorial on Win32 painting in general as some basic knowledge seems to be missing here. Get an understanding about what the "update region" is and how it interacts with the paint cycle.