ReadPixels - Screen.Width/Height in not maximized editor mode give unreliable results

In my current project, I'm taking a screenshot of the current camera view via texture.ReadPixels(). Code:

 Texture2D texture = new Texture2D(Screen.width, Screen.height, TextureFormat.ARGB32, false);
 texture.ReadPixels(new Rect(0f, 0f, Screen.width, Screen.height), 0, 0);

Everything is working fine all the time in maximized play inside the editor. When I'm logging Screen.Width() and Height it says 1920*1080 and everything is fine. However, when I'm not in the "Maximize On Play"-Mode I get the following error a lot:

[d3d11] attempting to ReadPixels outside of RenderTexture bounds! Reading (0, 0, 938, 420) from (937, 378)

Logging Screen.Width/Height tells me the resolution is 938 * 420. So I get why the exception is raised but I don't understand why Screen.Width/Height is giving a higher resolution back then what is actually displayed. Also from time to time I have to restart Unity and then everything works fine till I restart Unity again. Then the log says 1920*1080 even in not maximized mode.

Has anyone an idea how to solve this problem? It's really annoying to always test in maximized play-mode.