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Unglass makes things opaque when a window is maximized.
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New version: Unglass.zip — rather than fiddling with underlying windows, it simply switches off transparency by modifying theme color options. It also removes those pesky reflection stripes.
There is no GUI: just start the program. Kill it from the Task Manager.
Old versions: OldUnglass.zip and OldUnglassTaskbarOnly.zip
Changing color: Personalize → Desktop Background → Solid Colors, then switch back to your wallpaper (if any).