Visual issues with linked buttons

[Rudolf Palatický]

GTK buttons have many visual glitches. It looks like all default themes (Radiance, Ambiance and High Contrast) aren't compatible with GTK 3.18

- Open Nautilus, on the left tool bar click on "Documents", then click on "Home". After that the "Home" button on the top bar will look all rounded, where it shouldn't be in the side next to the adjacent button:
https://launchpadlibrarian.net/253125292/1-bad.png

- On the top bar, point the mouse over the "Home" button. After that the "Home" button looks OK:
https://launchpadlibrarian.net/253125315/2-ok.png

- Keep Nautilus open and don't touch it. Open System Settings and go back to Nautilus. After that on the top bar the "Home" button suddenly looks OK:
https://launchpadlibrarian.net/253125315/2-ok.png

- On the left tool bar, click on "Recent". After that the right side of the "Recent" button doesn't have all its corners rounded:
https://launchpadlibrarian.net/253125350/3-bad.png

- On the top bar, point the mouse over the "Recent" button. Then the button will look fine until you move the mouse pointer away.

This is just example, but there are many more visual issues.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: light-themes 14.04+16.04.20160324.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-18.34-generic 4.4.6
Uname: Linux 4.4.0-18-generic x86_64
ApportVersion: 2.20.1-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Apr 9 13:23:43 2016
InstallationDate: Installed on 2016-04-08 (0 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Beta amd64 (20160407)
PackageArchitecture: all
SourcePackage: ubuntu-themes
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.