Every once in a while the panel starts flickering a whole lot - it seems like it's being blanked out to background color and then the icons are repainted on top of it, and this occurs many times per second and lasts for many seconds. It happens on various occasions, one of them being when copying a large multi-GB file between hard disks (one of which is an NTFS spinning disk, so copy rate is about 50-60MB/s - in case that helps reproduce it in terms of cpu/io usage etc.). When opening and expanding the system tray notification to view the copy progress details, it causes the panel to flicker until the file copy is complete. It sometimes flickers also without opening the notification too. Interestingly, it's not the whole panel that flickers, but only it's right-hand side, including the clock, system tray, and two right-most taskbar buttons (if there are enough tasks open to cover the whole taskbar). The other taskbar buttons and the left side of the panel do not flicker. btw this started happening when upgrading to Kubuntu 15.04, which I think used KDE 5.3.0 originally. I hoped it would be fixed by 15.10 with KDE 5.4.0, but it still happens. Reproducible: Always
I have the exact same issue. Right side of panel (sometimes only 1/3 of it, sometimes more -> 2/3 of panel, it changes) I have Arch Linux with: Plasma 5.4.2 KF 5.15 Intel graphics (GLX or EGL, doesn't matter) Qt 5.5.1 It looks like it doesn't flicker with compositing disabled.
I have the same issue Kubuntu 15.10 Intel Graphics from the new Skylake processors.
Although it happens to me on NB with Intel graphics, I couldn't reproduce this issue on a NB with ATI (open-source RadeonSI driver). I'll try also with nouveau when I get the chance and let you know...
Forgot to mention I'm on Intel HD Graphics 4600. Coincidence?
Probably not. I've also tried with nvidia binary - no flickering. Can you please try to enable DRI3? For me it solved the flickering. At least I didn't spot it since. It can be enabled in xorg config file like that: Section "Device" Identifier "Intel Graphics" Driver "intel" Option "DRI" "3" EndSection Btw. it also solved some issues with EGL.
I can confirm the same behavior on plasma 5.5.3 (Arch Linux) with Intel integrated graphics. Martin's suggestion in https://bugs.kde.org/show_bug.cgi?id=354541#c5 seems to have solved the problem for me as a workaround.
Is this issue still relevant after updates?
After having these annoying flickering bugs for a long while, they went away and all graphics was smooth for a while. More recently in the past few months I started getting other sorts of flickering in the display again, although they seem related to the full display and not only to the panel. Specifically, I have not seen the issue during copying large files nor that only one side of the panel flickers. If I'd have to guess, I'd say the current issues I'm having are unrelated to this report, but who knows :-)
Given this only occurs on Intel graphics, I'm assuming this is a driver bug which has been fixed at some point.