AdjustWindowRectExForDpi doesn't seem to adjust client area size - c++

Question

While trying to create an application that correctly supports per-monitor DPI-awareness version 2, I encountered an issue where my application's window's client area size was incorrect when started on a monitor, where DPI scaling was enabled.
I leave choosing the appropriate location for the window up to Windows, so I can't know on which monitor the window is going to be created, thus I'm also unable to know the DPI I should scale for before the creation of the window.
The solution to this is that I obtain the DPI for the monitor, once the window has been created, using GetDpiForWindow and set the size, so it matches the client area size I desire. In this case I want the client area to be scaled - for example 300x150 client area when on a 125% display should be 375x187.
The DPI is obtained correctly (120 in my case), but using SetWindowPos means that I have to account for the window borders, titlebar, etc. For that purpose I use AdjustWindowRectExForDpi, which accounts for the DPI scaling of the window borders.
To my surprise, the resulting client area size still is 300x150, when the application is started on a DPI-scaled monitor. Starting the application on a non-DPI-scaled monitor and then moving it to one that is, results in a correct client area size.
Minimal example:
#include <Windows.h>
LRESULT CALLBACK startup_window_procedure(HWND window, UINT message, WPARAM w_param, LPARAM l_param)
{
switch (message)
{
case WM_DESTROY:
{
PostQuitMessage(0);
return 0;
}
case WM_DPICHANGED:
{
// Resize the window
RECT* new_rect = reinterpret_cast<RECT*>(l_param);
if (!SetWindowPos(window, nullptr, new_rect->left, new_rect->top, new_rect->right - new_rect->left, new_rect->bottom - new_rect->top, SWP_NOZORDER | SWP_NOACTIVATE))
{
return 1;
}
return 0;
}
}
return DefWindowProcW(window, message, w_param, l_param);
}
int CALLBACK wWinMain(HINSTANCE instance, HINSTANCE prev_instance, PWSTR cmd_line, int cmd_show)
{
constexpr auto window_class_name = L"example_dialog";
constexpr auto window_style = WS_OVERLAPPEDWINDOW;
// Enable per-monitor DPI-awareness version 2
if (!SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2))
{
return 1;
}
// Create the window
WNDCLASSEXW window_class;
window_class.cbSize = sizeof(window_class);
window_class.style = CS_HREDRAW | CS_VREDRAW;
window_class.lpfnWndProc = startup_window_procedure;
window_class.cbClsExtra = 0;
window_class.cbWndExtra = 0;
window_class.hInstance = instance;
window_class.hIcon = nullptr;
window_class.hCursor = nullptr;
window_class.hbrBackground = reinterpret_cast<HBRUSH>(COLOR_WINDOW + 1);
window_class.lpszMenuName = nullptr;
window_class.lpszClassName = window_class_name;
window_class.hIconSm = nullptr;
if (!RegisterClassExW(&window_class))
{
return 1;
}
HWND window = CreateWindowExW(0, window_class_name, L"Example window", window_style, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, nullptr, nullptr, instance, nullptr);
if (!window)
{
return 1;
}
UINT dpi = GetDpiForWindow(window);
// Actually set the appropriate window size
RECT scaled_size;
scaled_size.left = 0;
scaled_size.top = 0;
scaled_size.right = 300;
scaled_size.bottom = 150;
if (!AdjustWindowRectExForDpi(&scaled_size, window_style, false, 0, dpi))
{
return 1;
}
if (!SetWindowPos(window, nullptr, 0, 0, scaled_size.right - scaled_size.left, scaled_size.bottom - scaled_size.top, SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOMOVE))
{
return 1;
}
ShowWindow(window, SW_SHOWNORMAL);
// Message loop
MSG message;
int result;
while ((result = GetMessageW(&message, nullptr, 0, 0)) != 0)
{
if (result == -1)
{
return 1;
}
else
{
TranslateMessage(&message);
DispatchMessageW(&message);
}
}
return static_cast<int>(message.wParam);
}
The example requires at least Windows 10 1607 to run and Windows SDK 14393 to compile.
How do I properly scale the client area size, when the application is started on a DPI-scaled monitor?

Answer 1

AdjustWindowRectExForDpi doesn't adjust the client area size, though it does account for the increased size of window borders and the title bar, when the non-client area of a window is DPI-scaled. This means that you must scale the client area size yourself.
The example's code can be modified to scale the client area size manually like this:
// Calculate the scaling factor. 96 is the default Windows DPI, unless DPI scaling is enabled.
float scaling_factor = static_cast<float>(dpi) / 96;
RECT scaled_size;
scaled_size.left = 0;
scaled_size.top = 0;
scaled_size.right = static_cast<LONG>(300 * scaling_factor);
scaled_size.bottom = static_cast<LONG>(150 * scaling_factor);
if (!AdjustWindowRectExForDpi(&scaled_size, window_style, false, 0, dpi))
{
return 1;
}
if (!SetWindowPos(window, nullptr, 0, 0, scaled_size.right - scaled_size.left, scaled_size.bottom - scaled_size.top, SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOMOVE))
{
return 1;
}