Windows 2000非対応アプリを動かすとウインドウが透明になることがある理由が判明

R1599 より:

The reason why the window becomes transparent when running an application that is not compatible with Windows 2000 is related to how the operating system handles transparency and rendering of graphical elements.
In older versions of Windows like Windows 2000, the APIs (Application Programming Interfaces) used for graphic drawing, such as FillRect, did not support transparency. This means that when you draw using these APIs, the specified color appears as is, without any transparency.
However, newer APIs like Gdiplus::SolidBrush support transparency. These brushes use an alpha (α) channel to control transparency. When you specify an alpha value of 0, the brush becomes transparent, and nothing is drawn.
The problem occurs when incompatible applications with Windows 2000 use these transparent brushes. In Windows 2000, the operating system does not handle these transparent brushes correctly and initializes them to white by default. This means that even if nothing is drawn, the window appears to be filled with white, creating the illusion of a transparent window.
However, in later versions of Windows like Windows XP, this issue was fixed, and transparent brushes are initialized correctly. Therefore, even if an incompatible application uses these transparent brushes, the window does not appear transparent because it is not filled with white. you are capable to fix it ?