After updating to Xenforo 1.4.2 it mostly corrected the cursor appearing after replying to a thread in iOS8 but members are reporting it still is occurring.
Here's a screenshot from one of our members:
I think we might need to just call this a browser bug. The cursor isn't something that we have any direct control over. The workaround involved changing the focus on the editor when resetting it. This is probably about as far as we can control it directly. There isn't anything conditional as to how this code is executed, so I'm not sure why it would sporadically fail.
We don't typically distinguish between OS and browser bugs, however, it's worth noting that most of the bugs found specific to iOS 8 are specifically in Safari and cannot be reproduced in Chrome or other browsers.
Note that iOS only actually has one rendering engine as well. There aren't true alternative browsers (unlike on Android). They are generally just different "chrome" (as it's called) around the one iOS rendering engine (UIWebView). That said, Safari has access to optimizations that the UIWebView components don't.
[Edit: just to note that iOS8 does introduce WKWebView which has the JS optimization of Safari.]