Is it possbile to cover a win32 window with another transparent window without affecting user experience?

Question

let's take notepad.exe for example, is it possbile to create a same size transparent window on top of notepad main window without affecting user experience?

Answer 1

is it possbile to create a same size transparent window on top of notepad main window

It is pretty easy to FindWindow() the Notepad window, then GetWindowRect() its position and dimensions, and then place your custom window at the same location and size. You can use the WS_EX_LAYERED style with SetLayeredWindowAttributes() or UpdateLayeredWindow() to apply transparency/translucency to your window as needed.

Or, you might consider creating a layered window as a direct child of Notepad's window (Windows 8+ only), see:

Creating a Transparent Child window on top of non-transparent Parent Window ( win 32 )

without affecting user experience?

Not so easily, no. You might be tempted to use the WS_EX_TRANSPARENT window style, but be aware that WS_EX_TRANSPARENT is a lie, or at least not the entire truth (also this). Or you might be tempted to handle WM_NCHITTEST to return HTTRANSPARENT, but that gets complicated (also this).