Created attachment 609135 [details]
Screenshot: pdf viewer sidebar with unclear "pinned" icon.
The pin button to keep toggle sidebar auto-hide shows a blue circle when pinned, and a pushpin when not. Not obvious that the blue circle means "pinned."
Suggestion: use a depressed/un-depressed icon toggle to control pinned/unpinned, so it's always clear that the button controls pinned state, and the state is always clear if the sidebar is visible.
Liu, does the new UI of PDF Viewer 0.3.115 answer to your request?
Yes, pin/unpin of sidebar behavior is much more intuitive.