Emacs: how to remove "unread" tag while reading emails

[Gregor Zattler]

Dear notmuchers,

I had difficulties to reliably remove the "unread" tag from
messages.  Mostly I page through threads with the space bar and
all is well.  But when the beginning of the thread is already
collapsed and I "jump" in the middle of a message pressing space
bar does not remove the unread tag.  It's only removed when
*entering* the message via space bar from the previous message.
So the last press on space bar in the previous message jumps to
the next message and at the same time removes its unread tag.

This seems strange to me.  I would say the unread tag should be
removed when leaving the message with the last press on space
bar, indicating that one really paged trough the whole message
instead of only seeing the very beginning of it.

What’s the rationale to this behaviour?  Am I missing something?  

Thanks for your attention, gregor