As most are probably aware here by now, FXL epubs are hampered by the fact that no decent Windows/Linux/Android reader exists that handles these well. But that may be a thing of the past!
Tthe developers behind the defunct Readium (this was the only good alternative years ago, but development had to stop due to Google's changes to Chrome) have been developing a new epub reader for a while now, and 1.2 was released last April.
So far it seems to work quite well. State objects and animations work, although links between pages do not and have to be fixed after export. But this bodes well for FXL epubs on platforms other than the iBook reader app.
I still have to do more testing, in particular on Android (on that platform the reader is called "R2 Reader".
https://www.edrlab.org/software/thorium-reader/
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