This is partially a feature and partially a bug. Many of the keys act differently for short and long presses, in the case of the page bar in FBReader:
Quote:
# Page Bar Forward
Short Press - Next Page
Long Press - Redo
# Page Bar Backward
Short Press - Previous Page
Long Press - Undo
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So that is the "feature" part. The "bug" is that FBReader does not recover well from Redo/Undo (or Next/Prev Section) if there is no action to perform. It should just return, but it often does not.