Get Windows scale correctly (like 150%).

win32_scale.cpp

#pragma comment(lib, "user32")
#pragma comment(lib, "gdi32")
#pragma comment(lib, "shcore")

#define NOMINMAX
#define WIN32_LEAN_AND_MEAN
#include <ShellScalingApi.h>
#include <Windows.h>
#include <cstdio>

auto main() -> int {
  SetProcessDpiAwareness(PROCESS_PER_MONITOR_DPI_AWARE);
  HDC hdc = GetDC(HWND_DESKTOP);
  int dpi = GetDeviceCaps(hdc, LOGPIXELSY);
  ReleaseDC(HWND_DESKTOP, hdc);
  float scale = dpi * 1.0f / USER_DEFAULT_SCREEN_DPI;
  printf("%f\n", scale);
}

// This file uses #pragma comment(lib), it needs MSVC's cl.exe to compile
// cl /nologo win32_scale.cpp
// If you want to listen to DPI changes, see the WM_DPICHANGED event
// https://learn.microsoft.com/en-us/windows/win32/hidpi/wm-dpichanged

To render a window at high dpi, follow these steps:

int width = 1280, height = 720; // Your desired client rect size, I'm omitting scaling here.

int dpi = ...; // code above.
int style = WS_OVERLAPPEDWINDOW; // Should be the same as your window. But actually it seems doesn't matter.
int exStyle = 0;
RECT margin = {0};
AdjustWindowRectExForDpi(&margin, style, FALSE, exStyle, dpi); // The value of dpi is obtained by code above.
int marginX = margin.right - margin.left;
int marginY = margin.bottom - margin.top;

CreateWindowEx(
  exStyle, szWndClassName, title, style, x, y,
  width + marginX, height + marginY, // <--
  NULL, NULL, hInstance, NULL);

To enable dark mode, at least for the title bar and the system menu (right click on title bar), without self rendering those things, follow these steps:

// Enable dark mode system menu.
#pragma comment(lib, "uxtheme")
enum class PreferredAppMode {
  Default,
  AllowDark,
  ForceDark,
  ForceLight,
  Max
};
HMODULE hUxtheme = LoadLibrary("UxTheme.dll");
using TYPE_SetPreferredAppMode = PreferredAppMode(WINAPI *)(PreferredAppMode appMode);
static const TYPE_SetPreferredAppMode SetPreferredAppMode = (TYPE_SetPreferredAppMode)GetProcAddress(hUxtheme, MAKEINTRESOURCEA(135));
SetPreferredAppMode(PreferredAppMode::AllowDark);

// Enable dark title bar.
#pragma comment(lib, "dwmapi")
BOOL enabled = TRUE;
DwmSetWindowAttribute(hwnd, DWMWA_USE_IMMERSIVE_DARK_MODE, &enabled, sizeof(enabled));

Reference: https://source.chromium.org/chromium/chromium/src/+/main:base/win/dark_mode_support.cc