Highlighting flashcard words in a document is also something I'm not wild about, though that one's had quite a few requests
Another visual indicator, such as a superscripted symbol at the end of a word/term, could serve the same purpose without being distracting. (And all the better if you can toggle this on/off).
I'd be more inclined to keep them invisible in the document but to add an extra step to view their definitions
Just to play devil's advocate: isn't the reading experience disrupted every time the user has to tap? How does adding an extra tap create a less disruptive experience to the user? Although I didn't state it explicitly in what I wrote earlier, to me tapping is disruptive to the reading experience. The less I tap, the better.
In current/classic reader functionality, tapping is of course required to select an unknown word and reveal its definition. But in the "flashcard+reader" scenarios that I'm describing, there is a defined set of words with an extremely high probability of being selected by the user (e.g., flashcard words). How can Pleco quickly and easily present this information to the user in the least disruptive way possible?
With this split mode you're talking about, why does it even need to be split screen at all? Seems like it would be easier to simply show little correct / incorrect buttons in the reader bubble or in a toolbar somewhere.
I came up with the "split screen" solution because, to me, casting my glance from the lefthand "reader pane" to the righthand "flashcard pane" is faster and less disruptive than lifting my arm/hand and tapping twice to view the definition (or lifting my arm/hand to do anything, frankly). And I think that's even more true if the two panes are "scroll locked" such that as I scroll the reader's lefthand pane (again, that pane should be given the most real estate as possible) the relevant flashcards are displayed in the righthand pane.
I'm not really seeing the utility of the extra list of flashcards - feels like you're being taken out of the document context in precisely the situation where you're trying hard to stay within it.
We both want to minimize the disruption to the reading experience and just have different thoughts about what constitutes disruption, or how disruptive a given action is. As an end user, if I'm explicitly enabling the flashcard functionality in reader, then I'm willing to accept (or frankly want) a UI experience that (probably in your mind) takes away from the current "clean/pure" reader UI (basically a display of text) because I'm not using reader "just to read" anymore. (And if the flashcard functionality can be toggled, then I can always go back to the "clean" reader experience at will).
In the end, please do think about how you can minimize tapping (obviously I find tapping the most disruptive). So for example:
for example, a little message in the popup bubble telling you this is a flashcard word and offering to reveal it when tapped
So let's say there's no split screen UI, etc., and you still have to tap-select the flashcard word to see underlying information. OK I accept that. But I'd really want you to experiment with different UIs that could eliminate that second tap.
I know you're not keen on a split screen UI, so I tried to think of a solution that integrates some of this functionality without it, but which stays true to a "clean" reader UI experience: what about having a small visual indicator at the end of each paragraph? If it's present, than it means the paragraph contains flashcard words. If you tap the indicator, flashcard words within the paragraph are visually indicated (however you decide to do that), and a scrollable list of the cards is displayed. (I'd leave it up to you as to how to display the list: slide-out pane, pop-up window/bubble, anchored window at the bottom of the reader display, etc). I feel a solution like this still provides a very clean UI/experience, but gives a quick option for displaying flashcard information when needed, and even better, just for the text currently being read.