Proton with Gallium Nine patches fails to fit and/or stretch image to the screen if the game's resolution isn't set to native resolution. That applies to all Direct3D 9 games when Gallium Nine is active.
Company of Heroes in 1024x768 resolution on 1600x900 system:

Company of Heroes in 1600x900 resolution on 1600x900 system:

When the game's resolution is below the native resolution and doesn't fit the screen the screen as a result, navigating with mouse becomes harder since it cannot click menus as it supposed to. The workaround for this is returning to desktop with Alt+Tab, then get back to the game, finally positioning of the cursor becomes reliable.
Installed SDL packages on my system:
$ dpkg --get-selections | grep sdl
libsdl-image1.2:amd64 install
libsdl-image1.2:i386 install
libsdl-sound1.2:amd64 install
libsdl-stretch-0-3:amd64 install
libsdl-stretch-0-3:i386 install
libsdl-ttf2.0-0:amd64 install
libsdl-ttf2.0-0:i386 install
libsdl1.2debian:amd64 install
libsdl1.2debian:i386 install
libsdl2-2.0-0:amd64 install
libsdl2-2.0-0:i386 install
libsdl2-image-2.0-0:amd64 install
libsdl2-image-2.0-0:i386 install
libsdl2-ttf-2.0-0:amd64 install
libsdl2-ttf-2.0-0:i386 install
Installed mesa packages:
$ dpkg --get-selections | grep mesa
libd3dadapter9-mesa:amd64 install
libd3dadapter9-mesa:i386 install
libegl-mesa0:amd64 install
libegl-mesa0:i386 install
libegl1-mesa:amd64 install
libegl1-mesa:i386 install
libgl1-mesa-dri:amd64 install
libgl1-mesa-dri:i386 install
libgl1-mesa-glx:amd64 install
libgl1-mesa-glx:i386 install
libglapi-mesa:amd64 install
libglapi-mesa:i386 install
libglu1-mesa:amd64 install
libglu1-mesa:i386 install
libglx-mesa0:amd64 install
libglx-mesa0:i386 install
libosmesa6:amd64 install
libosmesa6:i386 install
mesa-utils install
mesa-va-drivers:amd64 install
mesa-va-drivers:i386 install
mesa-vdpau-drivers:amd64 install
mesa-vdpau-drivers:i386 install
mesa-vulkan-drivers:amd64 install
mesa-vulkan-drivers:i386 install
My specs:
https://gist.github.com/rea987/cfe3ac222c4cddfb29c3a3a45456812d
Proton with Gallium Nine patches fails to fit and/or stretch image to the screen if the game's resolution isn't set to native resolution. That applies to all Direct3D 9 games when Gallium Nine is active.
Company of Heroes in 1024x768 resolution on 1600x900 system:

Company of Heroes in 1600x900 resolution on 1600x900 system:

When the game's resolution is below the native resolution and doesn't fit the screen the screen as a result, navigating with mouse becomes harder since it cannot click menus as it supposed to. The workaround for this is returning to desktop with Alt+Tab, then get back to the game, finally positioning of the cursor becomes reliable.
Installed SDL packages on my system:
Installed mesa packages:
My specs:
https://gist.github.com/rea987/cfe3ac222c4cddfb29c3a3a45456812d