OK, just to recap. A few things, in combination, have dramatically improved (but not completely eliminated) the problem for me. Those are:
- System Preferences: turn off handoff (System Preferences -> General -> "Allow Handoff between this Mac and your iCloud devices"
- System Preferences: turn off smartquotes and dashes (System Preferences -> Keyboard -> Text -> "Use smart quotes and dashes"
- Mail.app Preferences: turn off auto detect and maintain account settings in each email account (Mail Preferences -> Accounts -> Advanced -> "Automatically detect and maintain account settings")
I'm noticing one other thing that I'd like to add to the pile -- a possible memory leak. I am composing a reply to a message and I've left it open while researching the lag issue. I'm watching Mail slowly ratchet it's memory consumption up from about 100 mB to 825 mB in the half hour that I've been researching. So I'm wondering if others are seeing the memory leak as well. Here's what I can report about the memory leak:
- It appears to increment at about 100k per second, very regular clock-like behavior
- It only happens when I am composing replies, it stops when I send the reply
- Mail doesn't reclaim the memory after the reply has been sent
I'm thinking that we're experiencing an interaction/combination type problem. I finally sent the reply (people were waiting for it) but I bet if I'd left it open, it would have eventually consumed a large proportion of memory on my machine which would have introduced a whole bunch of bad behaviors accompanied by spinning beach balls. So I'll be restarting Mail in just a second to clear that memory. And I'll also be using a text-editor to compose replies rather than the editor in Mail (so's to slow down the memory leak by having replies only open for a few seconds rather than half an hour or more).
For now, those three settings and periodic Mail-restarts have helped. Like others I'm experiencing the "cursor jump back to the previous line" problem while typing this post. As Sherlock Holmes would have said "this is a two-pipe problem Dr. Watson"