Fastmail support replied to my follow-up. They say:
Quote:
We've investigated and found this bug is happening in the Crosswalk framework that our app is built on, which unfortunately is no longer maintained. We need to move it to a different framework.
|
As well as suggesting using the web interface, they also pointed out that if you change the settings to compose plain text rather than rich text messages then the issue doesn't occur. (Settings -> Mail Preferences screen, under the Writing section.). Useful to know.