User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:53.0) Gecko/20100101 Firefox/53.0 Build ID: 20170524174214 Steps to reproduce: 1. Install Thunderbird on Kubuntu 17.04 2. Do NOT install any 3rd party icons for Thunderbird in /usr/share/icons 3. Use the Plasma Large Icons style for the Task Switcher (System Settings > Window Management > Task Switcher > Main > Large icons) 4. Hit alt-tab Actual results: The alt-tab window switcher displays Thunderbird's icon as an ugly scaled-up 48x48 icon. I've attached a screenshot showing the problem. Expected results: The 256x256 icon (default256.png) should be used instead, so that the large icon is pretty and not pixelated. The problem occurs because Thunderbird does not set _NET_WM_ICON_NAME to default256.png. Because of this, the window manager it not aware of the high-res icon, and is forced to display a scaled-up version of the largest icon it is aware of--the 48x48 version. This can be verified by running `xprop` and targeting Thunderbird's main window; you will see that the largest icon it knows about is only 48x48. For more information and context, see the following: https://bugs.kde.org/show_bug.cgi?id=351055 https://specifications.freedesktop.org/wm-spec/1.4/ar01s05.html

This might have been fixed in Thunderbird 59, thanks to bug 1414881. Could you verify if the latest nightly is fixed for you, pointedstick? Beta 59 is not yet ready.

Woohoo! Will check tonight.

Is there a reasonably user-friendly way to test a Thunderbird daily build without having to compile from source? I use the Flatpak nightly for Firefox and it was quite a lovely user experience.

Here you can find the 59 beta: https://www.mozilla.org/en-US/thunderbird/beta/all/ and here the Daily: http://archive.mozilla.org/pub/thunderbird/nightly/latest-comm-central/ Simply unpack the tar.gz file and start thunderbird in it.

Confirmed. Thunderbird is fixed too. Thanks so much! Feel free to mark the bug as RESOLVED FIXED, or a duplicate of the other bug.