Every Pass has a renderToScreen flag that defaults to false. Starting with version 6.11.0, the EffectComposer will automatically set this flag to let the last pass in the chain render to screen. If the user provides a pass that already renders to screen, this mechanism will be disabled.

You can also disable this behaviour manually by setting the autoRenderToScreen flag of the EffectComposer to false. Note that this flag will be set to true again if you call the reset method of the composer.