Question: I am developing an application which employs a self-drawn titlebar, which needs to mimic the system default titlebar. 

So how could I get the default titlebar height of an overloapped window in Windows?

Answer: 
Source code ported from Firefox:

```
// mCaptionHeight is the default size of the NC area at
// the top of the window. If the window has a caption,
// the size is calculated as the sum of:
//      SM_CYFRAME        - The thickness of the sizing border
//                          around a resizable window
//      SM_CXPADDEDBORDER - The amount of border padding
//                          for captioned windows
//      SM_CYCAPTION      - The height of the caption area
//
// If the window does not have a caption, mCaptionHeight will be equal to
// `GetSystemMetrics(SM_CYFRAME)`
int height = (GetSystemMetrics(SM_CYFRAME) + GetSystemMetrics(SM_CYCAPTION) +
    GetSystemMetrics(SM_CXPADDEDBORDER));
return height;

```

PS: the height is dpi-dependent.
One solution is to use the [AdjustWindowRectEx function](https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-adjustwindowrectex) which also computes other window borders width, and allows for window style variations:

```
RECT rcFrame = { 0 };
AdjustWindowRectEx(&rcFrame, WS_OVERLAPPEDWINDOW, FALSE, 0);
// abs(rcFrame.top) will contain the caption bar height

```

And for modern Windows (10+), there is the DPI-aware version:

```
// get DPI from somewhere, for example from the GetDpiForWindow function
const UINT dpi = GetDpiForWindow(myHwnd);
...
RECT rcFrame = { 0 };
AdjustWindowRectExForDpi(&rcFrame, WS_OVERLAPPEDWINDOW, FALSE, 0, dpi);
// abs(rcFrame.top) will contain the caption bar height

```