I have started migrating several Leap 42.2 machines to leap 42.3 but most users started complaining about kmail 17 on Leap 42.3
The problems are:
Kmail icon missing when there are unread messages
Unable to use shift delete
“Weird” behavior selecting messages
Links inside messages opening twice - for example, a youtube link opens the video on default browser and internally (the video audio starts to play)
I searched on Internet for solutions and discovered that theses bugs are related to QT 5.6 that was shipped with Leap 42.3, that these bugs do not occur on QT 5.8 and newer and the QT version won’t be upgraded in the life cycle of Leap 42.3.
that sounds more like a plasma 5 (kde applications) issue then a kernel issue
afaik at the moment that kernel has not been build for LEAP https://software.opensuse.org/package/kernel-default
you can always build it your self but…
it’s not a good idea especially if you have nvidia or amd graphic cards or any extra hardware or you are on production machines
personally I don’t use kmail and I haven’t noticed anything
maybe a better idea would be to migrate to thunderbird or open a bug report at over at kde who do the kmail development https://bugs.kde.org/
even at the opensuse bugzilla (you’d get better advice there too) https://bugzilla.opensuse.org/
