Use of arrow keys to move down/up in message list leads to crash

I meant "then up" not "the up".

Could you try fetching the latest test release: Hold down ⌥ when clicking “Check Now” in the Software Update preferences pane. You should get r5319.

Then allow MailMate to send some of these crash reports (see the General preferences pane if you do not already do this).

I tried reproducing it, but I couldn't. It might be some kind of race condition, but the crash reports might help me.

I just updated to r5319 and enabled crash reporting.
I'm still able to get it to crash, but it takes longer. In previous version, quickly arrowing down 10x then up 10x would cause it to crash. Now I need to do this up/down cycle maybe 3-4x before it crashes, which is an odd use case so perhaps it's solved.
Thanks for the quick attention.

On second thought, it's not so difficult to get it to crash doing this, even now. The easiest way to reproduce, is to press and hold down arrow for a couple seconds, then press and hold the up arrow. Repeat until crash occurs.
I'm on OS 10.10.5, if that helps.

I tried again with no luck. It might be related to the speed of the machine or the OS. In the latter case it could also be related to an issue in an Apple framework. I'm on Sierra. Around 6% of MailMate users are on 10.10 which might explain why I don't seem to have any similar reports. I'm afraid I don't have an install of Yosemite to test on myself (I've got everything else since 10.7).

It appears to be related to the message view. Do you use the standard 3-pane layout?

I use the widescreen layout. But I just tried it in 3 pane view and reproduced the error easily.
I appreciate your effort trying to reproduce/solve it. I can work around it.