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Johannes, Stefan, thank you both for your replies and explanations. I don't want to waste your time by going through the package code. If the behavior I see is a bug, that's enough of an explanation for me. I just thought that it's a known/expected thing and these I'd like to understand better. I'm ...

For this use case, i wouldn't use package titlesec , i would patch the original command.
Thanks a lot for this workaround and also for introducing me to showframe . But isn't this kind of customization exactly what titlesec is meant for? I would really like to understand what's going wrong there.

For a custom document type (based on article if this is relevant) I'm trying to redefine the way LaTeX typesets a \part . I want it to look more or less like a section with a larger font. For this I use titlesec . It works fine, except that LaTeX will insert additional space if the part start at the ...