Steps to Reproduce: 1. Use the 'Standard' layout from 'View - Layout' in the menu. Show the message view and adjust its size until there are six messages visible in the message list. Hide the message view. 2. Terminate the Claws Mail program; then start it again. 3. Show the message view. Actual Result: The message view takes up a larger amount of space such that only about 2.5 messages are visible in the message list. Expected Result: There would still be six messages visible in the message list. Note that the expected result is how 3.18.0 (gtk2) currently works, in case that is a helpful reference. Depending on how often one opens and closes Claws, this bug results in needing to do a lot of redundant resizing of the message view area unless one is okay with the default. Based on my use case, I've kept the priority of this bug at 'Normal', although I do realize that other use cases would indicate a 'Trivial' or 'Enhancement' priority.
*** Bug 4520 has been marked as a duplicate of this bug. ***
I can confirm that CM v4.1.1git11 (which includes this bug fix) now works as expected for me on three different systems: Linux Mint 20.3, Linux Mint Debian Edition, and Fedora 36.