This is a general list of improvements I feel like Spark Reader could use regarding interaction with the UI, to be implemented one day.
Keyboard shortcuts. Most games don't seem to respond to unmapped keys, perhaps these could be used to work with Spark Reader to scroll through words, definitions and so on. Being fully controllable by keyboard shortcuts could be great, especially with games that don't like how Spark Reader handles focus, but even some actions to aid users without a scroll wheel (like me when I'm stuck with my laptop's terrible trackpad) can be useful.
Click-drag word select support. For manual splitting, perhaps dragging from the start of the word to the end could put split points on both ends and automatically select the word. Right click and drag would do the same but open the right click menu on that word immediately, for quick export for example. I've come close to deciding to implement this one, but I still haven't gotten around to it.
A menu bar (like File, Edit, Help etc.) instead of the current '三' button on the top right. This would only appear when the mouse is over the furigana bar and have quick access to commonly changed settings, allow you to hook to VN windows and so on when these become features. With some of the plans I have, the current 三 menu is going to be pretty cluttered, and I haven't found myself using the button anyway; I've just been using the furigana right click shortcut.
This is a general list of improvements I feel like Spark Reader could use regarding interaction with the UI, to be implemented one day.