Currently, when I reply to a plain-text message that contains collapsed content (more than one level of replies within the body), the collapsed content isn't included in the reply. This should be configurable (or the display of messages should not be collapsed by default). Most of the times, I wish to include at least some of the thread's history in the reply, not just the immediately preceding message. In order to do that, I now need to manually expand the message by clicking on the 'ellipsis' buttons before I hit 'reply'.
Yes, the reply is always quoting exactly what is shown to the user. It's an interesting suggestion to have the quote collapsing parameters exposed to the user in some way. Patches (or even suggestions) are welcome. The implementation lives in src/UiUtils/PlainTextFormatter.cpp , the numeric parameters can be found there.
My suggestion would be to either not to collapse by default (that seems to me to be a less intrusive default than collapsing by default), or to make it configurable. I.e. to include an option (in the 'View' menu?) that would toggle automatic collapsing. At the very least, It'd be nice to have the possibility to expand *all* collapsed parts with one click/shortcut. As it is now, if I want to view the whole message, I need to go through it, clicking along to show deeper and deeper levels. To me, that seems very inconvenient.
Trojitá is no longer maintained, please switch to a maintained alternative like https://apps.kde.org/kmail2/ Sorry for the inconveniences.