Fake cursor always shown on top of all other windows.

cursor_on_top.cc

// Show a triangle at mouse position, always on top.
// Warning: This was coded in 40 minutes, so the quality isn't the greatest :shrug:
// Blame: Gynvael Coldwind (2021)
// Known issues:
// - The "cursor" doesn't change between a normal arrow / text arrow / etc. It's just a triangle.
// - There is a minor lag to the "cursor" changing position.
// - Size probably doesn't adjust to DPI. Didn't test.
// - The "cursor" is a pixel off - that's kinda a workaround to not block the actual cursor's hot point.
// How to compile:
//   g++ cursor_on_top.cc -Wall -Wextra -lgdi32 -o cursor_on_top.exe
// Just use mingw-w64 installer to get a compiler if you don't have one:
// https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/installer/mingw-w64-install.exe

#include <windows.h>
#include <stdio.h>

#define IDT_TIMER1 1001
#define W 12
#define H 20

LONG prevX = -1;
LONG prevY = -1;

POINT arrow[] = {
  { 1, 1 },
  { W, H },
  { 1, H },
  { 1, 1 }
};

HRGN arrowRgn;

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
  switch (uMsg) {
    case WM_CREATE: {
        SetTimer(hwnd, IDT_TIMER1, 17 /* 59 fps */, NULL);
        arrowRgn = CreatePolygonRgn(arrow, sizeof(arrow) / sizeof(arrow[0]), ALTERNATE);
        SetWindowRgn(hwnd, arrowRgn, TRUE);
      }
      break;

    case WM_TIMER: {
        POINT p;
        GetCursorPos(&p);
        if (p.x == prevX && p.y == prevY) {
          break;
        } else {
          prevX = p.x;
          prevY = p.y;
          MoveWindow(hwnd, p.x + 1, p.y + 1, W, H, TRUE);
        }
      }
      break;

    case WM_NCHITTEST:
      return HTNOWHERE;

    case WM_CLOSE:
      KillTimer(hwnd, IDT_TIMER1);
      DestroyWindow(hwnd);
      PostQuitMessage(0);
      break;

    default:
      return DefWindowProc(hwnd, uMsg, wParam, lParam);
  }

  return 0;
}

int main(void) {
  HINSTANCE hInstance = GetModuleHandle(NULL);

  WNDCLASS wc{};
  wc.lpfnWndProc   = WindowProc;
  wc.hInstance     = hInstance;
  wc.lpszClassName = "FakeMouseCursorClass";

  RegisterClass(&wc);

  HWND hwnd = CreateWindowEx(
      WS_EX_TOPMOST | WS_EX_NOACTIVATE /*| WS_EX_TRANSPARENT | WS_EX_LAYERED*/,
      "FakeMouseCursorClass",
      "FakeMouseCursor",
      WS_POPUP | WS_VISIBLE | WS_BORDER,
      50, 50, W, H,
      NULL,
      NULL,
      hInstance,
      NULL
  );


  ShowWindow(hwnd, SW_SHOW);

  MSG msg{};
  while (GetMessage(&msg,0,0,0) > 0) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }

  return 0;
}