Windows 11 rounded windows

unround.c

#include <stdbool.h>
#include <windows.h>
#include <dwmapi.h>

extern IMAGE_DOS_HEADER __ImageBase;
#define HINST_THISCOMPONENT ((HINSTANCE)&__ImageBase)

#define DWMWA_WINDOW_CORNER_PREFERENCE (33)

typedef enum {
    DWMWCP_DEFAULT = 0,
    DWMWCP_DONOTROUND = 1,
    DWMWCP_ROUND = 2,
    DWMWCP_ROUNDSMALL = 3,
} DWM_WINDOW_CORNER_PREFERENCE;

struct window {
	HWND window;
	bool destroyed;
	DWM_WINDOW_CORNER_PREFERENCE roundness;
};

static void handle_nccreate(HWND window, CREATESTRUCTW *cs)
{
	struct window *data = cs->lpCreateParams;
	SetWindowLongPtrW(window, GWLP_USERDATA, (LONG_PTR)data);

	DwmSetWindowAttribute(window, DWMWA_WINDOW_CORNER_PREFERENCE,
	                      &(DWM_WINDOW_CORNER_PREFERENCE) { data->roundness },
	                      sizeof(DWM_WINDOW_CORNER_PREFERENCE));
}

static LRESULT CALLBACK window_proc(HWND window, UINT msg, WPARAM wparam,
                                    LPARAM lparam)
{
	struct window *data = (void *)GetWindowLongPtrW(window, GWLP_USERDATA);
	if (!data) {
		if (msg == WM_NCCREATE)
			handle_nccreate(window, (CREATESTRUCTW *)lparam);
		return DefWindowProcW(window, msg, wparam, lparam);
	}

	switch (msg) {
	case WM_CLOSE:
		DestroyWindow(window);
		data->destroyed = true;
		return 0;
	case WM_DESTROY:
		PostQuitMessage(0);
		return 0;
	}

	return DefWindowProcW(window, msg, wparam, lparam);
}

static ATOM get_class(void)
{
	static ATOM cls = 0;
	if (cls)
		return cls;

	cls = RegisterClassExW(&(WNDCLASSEXW) {
		.cbSize = sizeof(WNDCLASSEXW),
		.lpfnWndProc = window_proc,
		.hInstance = HINST_THISCOMPONENT,
		.hCursor = LoadCursor(NULL, IDC_ARROW),
		.hbrBackground = (HBRUSH)(COLOR_BTNFACE + 1),
		.lpszClassName = L"dark-mode-test",
	});
	return cls;
}

struct window *window_create(wchar_t *title, DWM_WINDOW_CORNER_PREFERENCE roundness)
{
	struct window *data = calloc(1, sizeof(struct window));
	data->roundness = roundness;
	data->window = CreateWindowExW(WS_EX_APPWINDOW,
		(LPWSTR)MAKEINTATOM(get_class()), title,
		WS_OVERLAPPEDWINDOW | WS_SIZEBOX, CW_USEDEFAULT, CW_USEDEFAULT,
		350, 150, NULL, NULL, HINST_THISCOMPONENT, data);

	ShowWindow(data->window, SW_SHOWDEFAULT);
	UpdateWindow(data->window);

	return data;
}

void window_destroy(struct window *window)
{
	if (!window)
		return;
	if (!window->destroyed) {
		DestroyWindow(window->window);
		window->destroyed = true;
	}
	free(window);
}

int main()
{
	struct window *unround_wnd = window_create(L"DWMWCP_DONOTROUND", DWMWCP_DONOTROUND);
	struct window *round_wnd = window_create(L"DWMWCP_ROUND", DWMWCP_ROUND);
	struct window *roundsmall_wnd = window_create(L"DWMWCP_ROUNDSMALL", DWMWCP_ROUNDSMALL);

	MSG message;
	while (GetMessageW(&message, NULL, 0, 0)) {
		TranslateMessage(&message);
		DispatchMessageW(&message);
	}

	window_destroy(unround_wnd);
	window_destroy(round_wnd);
	window_destroy(roundsmall_wnd);
	UnregisterClassW((LPWSTR)MAKEINTATOM(get_class()), HINST_THISCOMPONENT);
	return message.wParam;
}