I would like to encourage use of this feature, as well as remind myself that it exists. I know in one part of my brain that the 
circle icon can be clicked and will expand into a text-entry field, but the part of my brain that edits posts apparently is distant from that one. Is there an easy way to expand it by default?
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I don’t know, but if someone who knows a bit about theme components comes across this topic, here are the script and template used for the button/input:
I suppose displayEditReason() could be executed when the composer opens 
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Ideally also still respecting this.canEdit…
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