I will give my answers inline the quotes from your post:
"if the user "misses" the arrow and click the header instead, the selected item is lost (unselected).
- Yes, and the new item is selected (this is the regular behavior).
"Is it possible to expand the "arrow functionality" (expand collapse without "deselecting") to the whole header?
- You say, that selecting/deselecting items is functionality that you use, so if the "selection functionality"
is removed from the header, how would you select items (the headers will only expand/collapse items)? I guess this explain, why it's not exactly possible to do so.
"By the way - I don't even need the arrows - most users hate to "fiddle" with these little parts - they just click the header.
- Well, this was added because big part of our clients requested it as a feature.
Hope this is helpful for you!
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