A while back, I described the problem I'm having in my guild-forums XPages app, which is that it very easily gets its environment out of whack, to the point where changing any design or data note from outside the XPages environment caused an "X is incompatible with X" ClastCastException. This improved gradually over time. At some point in 8.5.2, I started being able to modify data documents again without the problem. When 8.5.3 came out, it improved again: I can now replicate over changes from my development server to the production one without having to bounce HTTP on the production one. At that point, it became much less of a hassle - sure, I still had to re-save a Java class file every time I modified an XPage, but that was only in development, so I could deal.
However, I think I've found the proper solution. In 8.5.3 (I think), IBM added a custom editor for the xsp.properties file (which you can reach using the "Package Explorer" Eclipse view, in whatever.nsf/WebContent/WEB-INF). Many of the options are duplicates of what you see in the normal "Application Properties" editor (since they edit the same properties), but there are some nifty additional goodies. I won't go into them all, but I urge you to take a look. The important one here is in the "Timeouts" section of the "General" tab: "Refresh entire application when design changes" . That sounded perfect and, lo and behold, it seems to do what I want: once I turned that on, re-saving an XPage stopped causing the ClassCastException. I haven't given it a proper testing, so it might not be everything I'm looking for, but I'm thrilled at the initial results. Having to re-save a Java file for each change wasn't a HUGE problem, but it was a niggling one, and not having to jump through the hoop helps make the whole environment seem less rickety.