Bug 427315

SUMMARY
My Kontact window's width is most of the time 960 px. However, being in the mail component and selecting certain e-mails for viewing, Kontact increases the window automatically to 1070 px. This is annoying, as the window then overlaps partially with the window of the right half of my screen.

STEPS TO REPRODUCE
1. Use a resolution with a width of less than 2140 px (this is an assumption, I couldn't test that this is necessary)
2. Open Kontact in the mail view
3. Put it on the left half of the screen, put there with kwin's half-of-screen feature, as with normal window sizing you can't get it below 1070 px (I assume this is the root-cause of the bug!)
4. Select different e-mails, especially mails with long header entries (mail summary, long sender or multiple recipients)

OBSERVED RESULT
The window size gets increased to 1070 px when using Kontact.
However, using the geometrically identical setup with Kmail, the window size is not changed when viewing the same e-mails.

EXPECTED RESULT
The window width is not changed by viewing e-mails in Kontact (as it is already the case with Kmail).
The window can be sized with normal window sizing below a width of 1070 px (as it is already the case with Kmail).

Interestingly, I have not been able to enforce the maximum window width with Kwin window rules. Should I fill a separate bug for that one or is that the intended behavior in this case?

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux 
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2
Kernel Version: 5.8.0-2-amd64
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-6500 CPU @ 3.20GHz
Memory: 11,6 GiB

ADDITIONAL INFORMATION
This is currently my most annoying issue with Kontact. So Kudos to the developers for their hard work during the last years, as this is really peanuts compared to some older problems!