Why isn't SetWindowRgn working as it should? [duplicate] - c++

Question

I have some problems with making static controls rounded. I can't understand why SetWindowRgn doesn't work for a static control here. Also I've tried SelectClipRgn and it works. But nevertheless why SetWindowRgn doesn't? The Microsoft documentation states that
The system does not display any portion of a window that lies outside
of the window region.
The control must be rounded and clipped according to the documentation. But it is not. Here's my example of the problem:
#include <windows.h>
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE, PWSTR pCmdLine, int nCmdShow)
{
const wchar_t CLASS_NAME[] = L"Sample Window Class";
WNDCLASS wc = { };
wc.lpfnWndProc = WindowProc;
wc.hInstance = hInstance;
wc.lpszClassName = CLASS_NAME;
RegisterClass(&wc);
HWND hwnd = CreateWindowEx(
0, // Optional window styles.
CLASS_NAME, // Window class
L"Learn to Program Windows", // Window text
WS_OVERLAPPEDWINDOW | WS_CLIPCHILDREN, // Window style
// Size and position
CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
NULL, // Parent window
NULL, // Menu
hInstance, // Instance handle
NULL // Additional application data
);
if (hwnd == NULL)
{
return 0;
}
HRGN hrgnMain = CreateRoundRectRgn(0, 0, 200, 200, 5, 5);
int res = SetWindowRgn(hwnd, hrgnMain, TRUE);
ShowWindow(hwnd, nCmdShow);
HWND hControl = CreateWindow(L"Static", L"hello, world", WS_VISIBLE | WS_CHILD | SS_NOTIFY | SS_LEFTNOWORDWRAP,
20, 20, 40, 40, hwnd, NULL, hInstance, NULL);
HRGN hrgnControl = CreateRoundRectRgn(0, 0, 10, 10, 5, 5);
res = SetWindowRgn(hControl, hrgnControl, TRUE);
ShowWindow(hwnd, nCmdShow);
MSG msg = { };
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
switch (uMsg)
{
case WM_DESTROY:
PostQuitMessage(0);
return 0;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
FillRect(hdc, &ps.rcPaint, (HBRUSH)(COLOR_WINDOW + 1));
EndPaint(hwnd, &ps);
}
return 0;
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

Answer 1

Some builtin controls use CS_PARENTDC class style.
CS_PARENTDC sets the clipping region of the child window to that of the parent window. This conflicts with SetWindowRgn which wants to install custom region. Depending how your window is refreshed you could get different combinations of custom region working or not. For example if you resize parent window you can get your control refreshed partially with region set and partially without region.
Parent Display Device Contexts states that:
The system ignores the CS_PARENTDC style if the parent window uses a
private or class device context, if the parent window clips its child
windows, or if the child window clips its child windows or sibling
windows.
But it looks that setting only WS_CLIPCHILDREN for parent windows is not enough. Adding WS_CLIPSIBLINGS or WS_CLIPCHILDREN flags in control styles (even if you have only one child) triggers desired behavior.
HWND hControl = CreateWindow(
L"Static",
L"hello, world",
WS_CLIPSIBLINGS | WS_VISIBLE | WS_CHILD | SS_NOTIFY | SS_LEFTNOWORDWRAP,
20, 20, 40, 40,
hwnd,
NULL,
hInstance,
NULL);
Your code without WS_CLIPSIBLINGS
And with WS_CLIPSIBLINGS
Alternatively the CS_PARENT style could be removed by using GetClassLongPtr and SetClassLongPtr. Because CS_PARENT is used only to reuse clipping region it shouldn't have any other unexpected effects.
SetClassLongPtr(
hControl,
GCL_STYLE,
GetClassLongPtr( hControl, GCL_STYLE ) & ~CS_PARENTDC );