There are several UI elements that compete for arrow keys. For example:
- Navigate timeline (move through frames and layers, as arrow keys actually work)
- Move selection boundaries (actually Alt+arrow keys, we can use +Shift to move the selection through tiles)
- Move selection content (arrow keys when the selection is on)
- Move cel position/frame content (like Ctrl+mouse movement, this behavior is not available at the moment)
- Scroll (actually Space bar+arrow keys, we can use +Shift to scroll tiles)
- Move through range loop
- Move cursor pixel by pixel
- Move layers and frames in timeline
- Expand/collapse layer groups
We should have a way to assign arrow keys dynamically depending on what we want to do. Maybe a little arrows 
icon that can be dragged to the UI element that we want to control with arrow keys.
More information: Review arrow keys · Issue #558 · aseprite/aseprite · GitHub