Plasma panel displays closed window borders badly in Breeze theme at certain heights

[skierpage]

I've upgraded Kubuntu since 9.04 with little or no customization. Since upgrading to 16.04, the Plasma panel displays closed window "buttons" poorly, see attachment. They have weird gray shading over them. If I go into Plasma panel menu > Height and increase the size of the panel, the gray bits correctly resolve into a rounded rectangle border, but at smaller heights the gray borders appear on top of the inner content.

System Settings > Look and Feel says I'm using Breeze, with with no obvious customizations in Details. If I use other themes the weird rendering goes away and the text in the window buttons is tiny, so maybe the issue is I am using a larger font with a small-ish panel height. I'm using a 1920x1200 LCD panel.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: plasma-workspace 4:5.5.5.2-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-22.39-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Wed May 11 12:51:17 2016
ExecutablePath: /usr/bin/plasmashell
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/zsh
SourcePackage: plasma-workspace
UpgradeStatus: Upgraded to xenial on 2016-04-28 (13 days ago)
XsessionErrors:
 (process:4686): GLib-CRITICAL **: g_path_get_basename: assertion 'file_name != NULL' failed
 (process:5146): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
 (firefox:4595): Gtk-CRITICAL **: gtk_clipboard_set_with_data: assertion 'targets != NULL' failed
 (process:17720): GLib-CRITICAL **: g_path_get_basename: assertion 'file_name != NULL' failed

[Scarlett Gately Moore (scarlettmoore)]

This bug is very old, if it remains an issue please update the bug with info from a supported release.