please attach the output of
   qdbus org.kde.KWin /KWin supportInformation

try to run
    KWIN_USE_BUFFER_AGE=0 kwin_x11 --replace &

> qdbus org.kde.KWin /KWin supportInformation
KWin Support Information:
The following information should be used when requesting support on e.g. http://forum.kde.org.
It provides information about the currently running instance, which options are used,
what OpenGL driver and which effects are running.
Please post the information provided underneath this introductory text to a paste bin service
like http://paste.kde.org instead of pasting into support threads.

==========================

Version
=======
KWin version: 5.4.2
Qt Version: 5.5.0
Qt compile version: 5.5.0
XCB compile version: 1.11.1

Operation Mode: X11 only

Build Options
=============
KWIN_BUILD_DECORATIONS: yes
KWIN_BUILD_TABBOX: yes
KWIN_BUILD_ACTIVITIES: yes
HAVE_WAYLAND: yes
HAVE_WAYLAND_EGL: yes
HAVE_WAYLAND_CURSOR: yes
HAVE_XKB: yes
HAVE_INPUT: yes
HAVE_DRM: yes
HAVE_GBM: yes
HAVE_X11_XCB: yes

X11
===
Vendor: The X.Org Foundation
Vendor Release: 11702000
Protocol Version/Revision: 11/0
SHAPE: yes; Version: 0x11
RANDR: yes; Version: 0x14
DAMAGE: yes; Version: 0x11
Composite: yes; Version: 0x4
RENDER: yes; Version: 0xb
XFIXES: yes; Version: 0x50
SYNC: yes; Version: 0x31
GLX: yes; Version: 0x0

Decoration
==========
Plugin: org.kde.breeze
Theme: 
Blur: 0
onAllDesktopsAvailable: true
alphaChannelSupported: true
closeOnDoubleClickOnMenu: false
decorationButtonsLeft: 0, 2
decorationButtonsRight: 6, 3, 4, 5
borderSize: 3
gridUnit: 10
font: Oxygen-Sans,10,-1,0,50,0,0,0,0,0
smallSpacing: 2
largeSpacing: 10

Options
=======
focusPolicy: 0
nextFocusPrefersMouse: false
clickRaise: true
autoRaise: false
autoRaiseInterval: 0
delayFocusInterval: 0
shadeHover: false
shadeHoverInterval: 250
separateScreenFocus: false
placement: 4
focusPolicyIsReasonable: true
borderSnapZone: 10
windowSnapZone: 10
centerSnapZone: 0
snapOnlyWhenOverlapping: false
rollOverDesktops: true
focusStealingPreventionLevel: 1
legacyFullscreenSupport: false
operationTitlebarDblClick: 5000
operationMaxButtonLeftClick: 5000
operationMaxButtonMiddleClick: 5015
operationMaxButtonRightClick: 5014
commandActiveTitlebar1: 0
commandActiveTitlebar2: 30
commandActiveTitlebar3: 2
commandInactiveTitlebar1: 4
commandInactiveTitlebar2: 30
commandInactiveTitlebar3: 2
commandWindow1: 7
commandWindow2: 8
commandWindow3: 8
commandWindowWheel: 31
commandAll1: 10
commandAll2: 3
commandAll3: 14
keyCmdAllModKey: 16777251
showGeometryTip: false
condensedTitle: false
electricBorderMaximize: true
electricBorderTiling: true
electricBorderCornerRatio: 0.25
borderlessMaximizedWindows: false
killPingTimeout: 5000
hideUtilityWindowsForInactive: true
inactiveTabsSkipTaskbar: false
autogroupSimilarWindows: false
autogroupInForeground: true
compositingMode: 1
useCompositing: true
compositingInitialized: true
hiddenPreviews: 1
unredirectFullscreen: false
glSmoothScale: 2
colorCorrected: false
xrenderSmoothScale: false
maxFpsInterval: 16666666
refreshRate: 0
vBlankTime: 6000000
glStrictBinding: false
glStrictBindingFollowsDriver: true
glCoreProfile: false
glPreferBufferSwap: 101
glPlatformInterface: 1

Screen Edges
============
desktopSwitching: false
desktopSwitchingMovingClients: false
cursorPushBackDistance: 1x1
timeThreshold: 150
reActivateThreshold: 350
actionTopLeft: 0
actionTop: 0
actionTopRight: 0
actionRight: 0
actionBottomRight: 0
actionBottom: 0
actionBottomLeft: 0
actionLeft: 0

Screens
=======
Multi-Head: no
Active screen follows mouse:  no
Number of Screens: 1

Screen 0:
---------
Name: LVDS
Geometry: 0,0,1366x768
Refresh Rate: 60.0204


Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD PALM (DRM 2.42.0, LLVM 3.7.0)
OpenGL version string: 3.0 Mesa 11.0.2
OpenGL platform interface: GLX
OpenGL shading language version string: 1.30
Driver: R600G
GPU class: EVERGREEN
OpenGL version: 3.0
GLSL version: 1.30
Mesa version: 11.0.2
X server version: 1.17.2
Linux kernel version: 4.1.12
Direct rendering: Requires strict binding: no
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace:  yes

Loaded Effects:
---------------
zoom
slidingpopups
kwin4_effect_login
slide
screenshot
minimizeanimation
kwin4_effect_translucency
desktopgrid
kwin4_effect_windowaperture
kwin4_effect_fade
kwin4_effect_maximize
presentwindows
highlightwindow
kwin4_effect_dialogparent
blur
contrast
logout
dashboard
startupfeedback
screenedge
kscreen

Currently Active Effects:
-------------------------
blur
contrast

Effect Settings:
----------------
zoom:
zoomFactor: 1.2
mousePointer: 0
mouseTracking: 0
enableFocusTracking: false
followFocus: true
focusDelay: 350
moveFactor: 20
targetZoom: 1

slidingpopups:
fadeInTime: 150
fadeOutTime: 250

kwin4_effect_login:

slide:

screenshot:

minimizeanimation:

kwin4_effect_translucency:

desktopgrid:
zoomDuration: 300
border: 10
desktopNameAlignment: 0
layoutMode: 0
customLayoutRows: 2
usePresentWindows: true

kwin4_effect_windowaperture:

kwin4_effect_fade:

kwin4_effect_maximize:

presentwindows:
layoutMode: 0
showCaptions: true
showIcons: true
doNotCloseWindows: false
ignoreMinimized: false
accuracy: 20
fillGaps: true
fadeDuration: 150
showPanel: false
leftButtonWindow: 1
rightButtonWindow: 2
middleButtonWindow: 0
leftButtonDesktop: 2
middleButtonDesktop: 0
rightButtonDesktop: 0

highlightwindow:

kwin4_effect_dialogparent:

blur:
blurRadius: 12
cacheTexture: true

contrast:

logout:
useBlur: true

dashboard:
brightness: 0.5
saturation: 0.5
blur: false

startupfeedback:
type: 1

screenedge:

kscreen:

Created attachment 95383 [details]
kwin support info output

I'm experiencing something like this too (openSUSE Leap 42.1), not sure it's really the same.
Whenever I hit Alt+Tab (Large Icons switcher) only shortly (i.e. that it does not really have the time to show) it's that the window switcher (tabbox) flickers shortly (while it fades?).
Doing a
KWIN_USE_BUFFER_AGE=0 kwin_x11 --replace &
does not seem to change anything

With every invocation of the window switcher, a new line shows up in the terminal:
QXcbConnection: XCB error: 3 (BadWindow), sequence: 43317, resource id: 81790595, major code: 20 (GetProperty), minor code: 0

run "kcmshell5 kwineffects", configure the blur effect and disable caching. Problem gone? Wouldn't sound the same to me (in case)

I am using opensuse 42.1 now and it seems that this problem only occurs on the first login of plasma. After logging out and entering plasma again, the flashing disappears.
I have tried to disable the "Save intermediate rendering results" in the blur effect and the problem totally disappears even on the first login.

You're experiencing bug #307112 (and I wanted to know from Thomas M. whether he does)
Afawcs, this is *not* the same bug.

(In reply to Thomas Lübking from comment #7)
> You're experiencing bug #307112 (and I wanted to know from Thomas M. whether
> he does)
> Afawcs, this is *not* the same bug.

Sorry for my mistake, I rebooted several times and found the flashing artifacts appear again even after I totally turned off the blur effect.
Now the problem appears randomly when I login, and it seems has nothing to do with the blur effect.

Then please see comment  #1

I think you are right: it's not the same bug, because it is only the box switch which flashes multiple times, not a window. Unfortunately, I cannot contribute insight anymore because I had to go back to Plasma 4 on my production machine because of the frequent crashes/freezes/glitches in the Plasma 5.4.2 packages shipped with openSUSE Leap 42.1.

qdbus org.kde.KWin /KWin supportInformation:
See here: https://paste.kde.org/poamu597e

Running "KWIN_USE_BUFFER_AGE=0 kwin_x11 --replace &" solves the problem but I think I should test it for more times.

Created attachment 95472 [details]
support info from yjcoshc

Please always attach information to the bug an never use paste services, as the data will be lost there at some point in time.

After doing more test, I found my problem was related to "Painting blocks for vertical retrace" in qdbus org.kde.KWin /KWin supportInformation.
When it turned to "yes", the flashing happened. When it turned to "no" the problem disappeared.
I think my problem is https://bugs.kde.org/show_bug.cgi?id=346275.

Hardly, triple buffering misdetection may cause stutter, but not "flashing artifacts" - and the bug is (likely) an nvidia specific issue (and notably occurs *with* doublebuffering)

There may be an unknown relation, though, where the stalled (overloaded) buffer causes artifacts due to re-using the "wrong" old buffer (buffer_age support)

You may try to skip detection w/
KWIN_USE_BUFFER_AGE=0 KWIN_TRIPLE_BUFFER=1 kwin_x11 --replace &

"and the bug is (likely) an nvidia specific issue"

I have an AMD Radeon HD 6250

The KWIN_USE_BUFFER_AGE=0 part seemed to help the issue, but it came back after a few minutes. I haven't extensively tested it past that though.

(In reply to illumilore from comment #15)
> I have an AMD Radeon HD 6250

I know, I was referring to bug #346275 mentioned on comment #13

> after a few minutes. I haven't extensively tested it past that though.
What means it's not about buffer age


However, just like yjcoshc your support info indicate blocking swaps, but during the detection phase we do not treat it like that (and you suggested it would only re-occur after a short time) so overriding the detection like

KWIN_USE_BUFFER_AGE=0 KWIN_TRIPLE_BUFFER=1 kwin_x11 --replace &

   and maybe just

KWIN_TRIPLE_BUFFER=1 kwin_x11 --replace &

might make a difference?

http://askubuntu.com/questions/672007/flickering-in-kde-plasma-5-4
Are you talking about this issue? The 3rd method in the selected answer seems to have fixed opengl 2.0 for me as a working work around.
Using Ubuntu 16.04

btw possibly a duplication of https://bugs.kde.org/show_bug.cgi?id=340489

ATI M7740 FireGL card (I know pretty old) and I have the same issue.  All fixes in that askubuntu link do not work for me.

I can confirm the problem and have found a fix.  I have an AMD C-60 APU and am running plasma 5.8 on Neon / Linux 4.4 kernel.  I am running off of the stock Mesa radeon r-600 driver.  What I found is that the flickering would not be as common in the first minute or so but then is more pronounced when viewing videos or the alt-tab desktop effects it is on both the OpenGL 2.0  and  3.1 versions.  I however turned the V-Sync to never and haven't had any problems with the flickering since.  So the Bug is related to the V-Sync code.

This issue report is quite old. Can you please confirm, that it still persists with KDE 5.23?

I haven't experienced this problem in a long time. So from my point of view this issue can be closed.