Claws Mail under KDE - message list dims when main window loses focus

[Little Girl]

The text in the message list pane in Claws Mail is greyed-out when the Claws Mail main window loses focus, either by opening a child window within Claws Mail or by putting the focus onto any other window. The normal text-color is only restored by leaving the current folder that has the greyed-out messages and then returning to it.

Here is an animated GIF file showing the behavior:

https://imgur.com/a/IQ97IVY

Note: I forgot to demonstrate, in that animated GIF, that closing the Notes window that triggered the issue doesn't restore the text color that was lost on opening it.

In that GIF, the Inbox is selected and the message list contains messages with colored text. I then open the Notes widget, which causes Claws Mail to lose focus and the message list text to be greyed-out. I then leave the Inbox and open the Sent folder for a moment and then leave the Sent folder and open the Inbox folder again, at which point, the text color is restored.

The environment that this is occuring in is:
 * Kubuntu release: Kubuntu 22.04 LTS
 * KDE Plasma Version 5.24.4
 * KDE Frameworks Version: 5.92.0
 * Qt Version: 5.15.3
 * Graphics Platform X11
 * Theme: Breeze
 * GTK Theme: Default
 * Claws Mail version: 4.0.0

This is on a bare-metal full installation (not an upgrade) of Kubuntu on a desktop computer. I used the Muon package manager to install Claws Mail.

I would expect the text color of the message list to be unaffected by the loss of focus of the Claws Mail main window. Or, if that's unavoidable, I would expect the colors to be restored automatically when the Claws Mail main window again regains focus.

It's possibly rather significant that all other GTK programs on my system are behaving normally and do not exhibit this behavior. The author of Claws Mail, however, suggests that a KDE setting is causing this issue:

https://lists.claws-mail.org/pipermail/users/2022-May/030087.html

As a result:
 * I've tried switching from the Default theme that's provided for GTK applications to the Emacs and Breeze themes that are offered for GTK applications with no improvement.
 * I've also tried fiddling with enabling and disabling various settings in the System Settings -> Workspace Behavior -> Desktop Effects section with no improvement.
 * I've also tried creating and fiddling with various settings in System Settings -> Window Management -> Window Rules -> Window settings for claws-mail with no improvement.

Any suggestions for what I can try next would be appreciated.