Trying to emerge gtk+:3 with pixman[-abi_x86_32] fails on config with: Package 'pixman-1', required by 'cairo', not found. Portage should suggest to enable abi_x86_32 for pixman prior to the emerge.
This is not a bug in gtk+. gtk:3[abi_x86_32] depends on cairo[abi_x86_32]; cairo[abi_x86_32] depends on pixman[abi_x86_32]. So everything works beautifully as long as users don't do strange things like breaking 32-bit cairo behind portage's back by disabling abi_x86_32 on cairo's dependencies. If you manually disabled abi_x86_32 on some libraries considering the effects - it means you broke other libraries depending on them, now you get to keep the pieces. You could have achieved the same breakage by completely unmerging pixman and then wondering why gtk+ doesn't build and doesn't run :) To fix your system, try running "emerge world --deep --pretend --verbose --tree" and see what packages need their flags to be changed.
