jls_legalize: If you first pop up the application menu with a right-click, then the key bindings listed in the menu work, at least for me.
But it is true that the main image window will not respond to the same key bindings if the right-click menu is not visible. flPicSee makes no explicit promise that the menu keybindings will work without the menu, but I know convention in MS Windows and perhaps other Qt and Gtk applications in Linux is to do so.
I have been working on other FLTK mini-projects lately (Flit, flwm alterations, flwget), so flPicSee has not seen much new development, but I can add better keybinding to the to-do list in flPicSee. While working on Flit, I learned how to do direct key event handling in FLTK, so I am better prepared to add direct key-binding in flPicSee.
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Mike