Window borders for apps using Client Side Decorations will soon look much better on Ubuntu.
Code implementing window corner radius support is finally headed to Unity.
If merged, the fix will solve the black corner issue affecting the appearance of every GTK3 application that uses CSD header bars running under Compiz & Unity. Squared-off corners have been a feature of CSD windows on Unity since Ubuntu 15.04.
Several default apps in Ubuntu 16.04 LTS use CSD (also known as GTKHeaderBar), including the Disks utility, the Deja Dup backup tool (thanks Aaron!) as well as the brand new desktop Calendar app that debuts in Xenial Xerus.
Other popular applications affected by black square window corners include GNOME Photos, Maps, Music & Weather apps, the Corebird Twitter client and GNOME Tweak Tool.
Applications that don’t natively use CSD (or that Ubuntu has patched to not use it, e.g., Nautilus, Videos) are not affected by the issue.
Fix Coming Very Soon
The window corner fix has been proposed for merging into Unity. It’s described as
DecoratedWindow: move the shadow under the window if we've a client-side decorated window with corners
Once merged, and a new version of Unity pushed out to 16.04 testers, app window corners should once again render correctly, i.e. appear curved, not topped by small black triangles.
Will you be pleased to see this bug fixed?
Annnnnnd …nope. Although the fix mentioned above has landed in Xenial-Proposed it does not fix the issue of black triangles.
The latest update to the Ubuntu Light Themes package does, indeed, fix this issue for most GNOME 3 apps – but not the new GNOME Calendar tool, bizarrely.