是什么引起两个相等的后续SetWindowPos（）调用来设置不同的窗口大小？

Question

As part of a project I'm working on, I'd like to highlight regions of the desktop. Right now I implement this using a translucent red top level window, ie I

1. Create a custom window class with a red background brush.
2. Create a window of my custom class which has the WS_EX_LAYERED style set.
3. Call SetLayeredWindowAttributes to make the window 50% translucent.

It mostly works quite well, but I noticed a fairly peculiar interaction between my overlay window and the SetWindowPos function: when passing widths or heights to SetWindowPos which are smaller than 32 resp. 39 pixels, the first call to SetWindowPos will actually make the window larger than requested but subsequent calls work as expected. Here's a small sample program which demonstrates the issue - it creates the overlay window in the top left corner of the desktop and then calls SetWindowPos twice with a one second delay. Notice how the red rectangle is first about square and then shrinks vertically.

#include <windows.h>

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow )
{
    // Register custom class for overlay window, forcing red background
    WNDCLASSW overlayClassDef = {
        0,
        DefWindowProcW,
        0,
        0,
        hInstance,
        NULL,
        NULL,
        ::CreateSolidBrush( RGB( 255, 0, 0 ) ),
        NULL,
        L"Overlay_Window"
    };

    ATOM overlayClass = ::RegisterClassW( &overlayClassDef );

    // Create overlay window using 'layered' flag to enable making it
    // translucent
    HWND m_overlay = ::CreateWindowExW(
        WS_EX_LAYERED | WS_EX_NOACTIVATE,
        (LPCWSTR)overlayClass,
        NULL,
        0,
        0,
        0,
        0,
        0,
        NULL,
        NULL,
        hInstance,
        NULL
    );

    // Strip WS_BORDER and WS_DLGFRAME styles to get perfectly flat window; these
    // styles appear to get added by default for toplevel windows.
    int windowStyle = ::GetWindowLongPtr( m_overlay, GWL_STYLE );
    windowStyle &= ~WS_BORDER;
    windowStyle &= ~WS_DLGFRAME;
    ::SetWindowLongPtr( m_overlay, GWL_STYLE, windowStyle );

    // Show the window and make it 50% translucent
    ::ShowWindow( m_overlay, SW_SHOWNA );
    ::SetLayeredWindowAttributes( m_overlay, 0, 127, LWA_ALPHA );

    // Set the position to 100/100 (50x20 pixels); the window on
    // screen becomes higher than 20 pixels though!
    ::SetWindowPos( m_overlay, HWND_TOPMOST, 100, 100, 50, 20, SWP_NOACTIVATE);
    ::Sleep( 1000 );

    // Set the position once more -- this time, the window shrinks
    // to 20 pixels vertically.
    ::SetWindowPos( m_overlay, HWND_TOPMOST, 100, 100, 50, 20, SWP_NOACTIVATE);
    ::Sleep( 1000 );

    // Releasing resources omitted for brevity
    return 0;
}

Some observations regarding this behaviour:

1. The effect does not occur if either WS_BORDER or WS_DLGFRAME is set in the window flags; however, I'd like to clear both flags to get a perfectly flat window.
2. The effect does not seem to occur horizontally when using widths >= 32 pixels
3. The effect does not seem to occur vertically when using heights >= 39 pixels
4. This can also be reproduced when not using a custom window class but rather instantiating the built-in STATIC class.

Is there maybe a magic minimum window size close to 32 pixels which has to be treated specially?

Answer 1

After you update window style you need to make sure that window size and frame metrics get recalculated. In order to do so you need to manually call SetWindowPos with SWP_FRAMECHANGED flag right after updating visual style:

::SetWindowLongPtr( m_overlay, GWL_STYLE, windowStyle );
::SetWindowPos(m_overlay, NULL, 0, 0, 0, 0, SWP_NOACTIVATE | SWP_FRAMECHANGED | SWP_NOMOVE | SWP_NOSIZE | SWP_NOOWNERZORDER | SWP_NOZORDER | SWP_NOREPOSITION | SWP_NOREDRAW);