I see that your video driver is from 6.Jun 2018. Maybe an Ubuntu driver works better. See here: https://www.nvidia.com/Download/driverResults.aspx/135161/en-us . Flickering playback is mostly a problem with the video driver. 

If you open a project from the previous version delete your cache. See here: https://community.kde.org/Kdenlive/Configuration

Created attachment 119636 [details]
attachment-14592-0.html

 Hi emohr, thanks for the suggestion but I am quite reluctant to change video drivers on a hunch on my production system when everything but 19.04 works fine with no flicker whatsoever. 

BTW, I did not open an old project with 19:04. I started a completely new one (which I have now deleted). 

Another point I should have made clearer: it is ONLY the project/clip monitor window content that 
is flickering, everything else, including the rest of Kdenlive's UI is fine.
I just completed my latest video using Kdenlive 18.12.1b appimage with no problems or flickering, so I have a workaround 

Best Regards

    On Thursday, 25 April 2019, 17:38:30 BST, emohr <bugzilla_noreply@kde.org> wrote:  
 
 https://bugs.kde.org/show_bug.cgi?id=406891

emohr <fritzibaby@gmx.net> changed:

          What    |Removed                    |Added
----------------------------------------------------------------------------
                CC|                            |fritzibaby@gmx.net
              Flags|                            |timeline_corruption+

--- Comment #1 from emohr <fritzibaby@gmx.net> ---
I see that your video driver is from 6.Jun 2018. Maybe an Ubuntu driver works
better. See here:
https://www.nvidia.com/Download/driverResults.aspx/135161/en-us . Flickering
playback is mostly a problem with the video driver. 

If you open a project from the previous version delete your cache. See here:
https://community.kde.org/Kdenlive/Configuration

I can confirm this bug on my system:

Arch Linux 5.0.10-1-ck
Kdenlive 19.04.0-3
mlt version 6.14.0

Video card NVIDIA Corporation GK107M [GeForce GT 750M Mac Edition] (rev a1)

Video driver nouveau

Swaywm (wayland)

This bug was not present in earlier version. It is purely limited to the monitor screen, which flickers a lot, or sometimes stops working entirely.

Output from starting the project from terminal:

❯ kdenlive Vilnius.kdenlive
mlt_repository_init: failed to dlopen /usr/lib/mlt/libmltsdl.so
  (libSDL_image-1.2.so.0: cannot open shared object file: No such file or directory)
"gtkrescale" is blacklisted
"resample" is blacklisted
WARNING : Fails to parse  "glsl.manager"
WARNING : Fails to parse  "movit.convert"
WARNING : Fails to parse  "movit.crop"
WARNING : Fails to parse  "movit.resample"
WARNING : Fails to parse  "movit.resize"
WARNING : Fails to parse  "telecide"
"frei0r.3dflippo" is blacklisted
"frei0r.bluescreen0r" is blacklisted
"frei0r.bw0r" is blacklisted
"frei0r.gamma" is blacklisted
"frei0r.invert0r" is blacklisted
"frei0r.rgbsplit0r" is blacklisted
"frei0r.transparency" is blacklisted
"frei0r.vertigo" is blacklisted
WARNING : Fails to parse  "jack"
"jackrack" is blacklisted
"qtext" is blacklisted
"motion_est" is blacklisted
WARNING : Fails to parse  "crop_detect"
"rgblut" is blacklisted
"spot_remover" is blacklisted
"text" is blacklisted
"timer" is blacklisted
"videostab" is blacklisted
"videostab2" is blacklisted
WARNING : Fails to parse  "deinterlace"
WARNING : Fails to parse  "avcolour_space"
WARNING : Fails to parse  "avcolor_space"
WARNING : Fails to parse  "avdeinterlace"
WARNING : Fails to parse  "swscale"
"avfilter.abench" is blacklisted
"avfilter.acompressor" is blacklisted
"avfilter.adelay" is blacklisted
"avfilter.aecho" is blacklisted
"avfilter.aemphasis" is blacklisted
"avfilter.aeval" is blacklisted
"avfilter.afade" is blacklisted
"avfilter.afftfilt" is blacklisted
"avfilter.agate" is blacklisted
"avfilter.ametadata" is blacklisted
"avfilter.arealtime" is blacklisted
"avfilter.ashowinfo" is blacklisted
"avfilter.channelmap" is blacklisted
"avfilter.chorus" is blacklisted
"avfilter.earwax" is blacklisted
"avfilter.volume" is blacklisted
"avfilter.volumedetect" is blacklisted
"avfilter.ass" is blacklisted
"avfilter.atadenoise" is blacklisted
"avfilter.avgblur" is blacklisted
"avfilter.bbox" is blacklisted
"avfilter.bench" is blacklisted
"avfilter.blackdetect" is blacklisted
"avfilter.blackframe" is blacklisted
"avfilter.boxblur" is blacklisted
"avfilter.bwdif" is blacklisted
"avfilter.chromakey" is blacklisted
"avfilter.colorkey" is blacklisted
"avfilter.colormatrix" is blacklisted
"avfilter.colorspace" is blacklisted
"avfilter.convolution" is blacklisted
"avfilter.crop" is blacklisted
"avfilter.cropdetect" is blacklisted
"avfilter.curves" is blacklisted
"avfilter.datascope" is blacklisted
"avfilter.dctdnoiz" is blacklisted
"avfilter.deband" is blacklisted
"avfilter.deflate" is blacklisted
"avfilter.deinterlace_vaapi" is blacklisted
"avfilter.deshake" is blacklisted
"avfilter.despill" is blacklisted
"avfilter.doubleweave" is blacklisted
"avfilter.drawbox" is blacklisted
"avfilter.drawgraph" is blacklisted
"avfilter.drawgrid" is blacklisted
"avfilter.drawtext" is blacklisted
"avfilter.elbg" is blacklisted
"avfilter.eq" is blacklisted
"avfilter.fade" is blacklisted
"avfilter.field" is blacklisted
"avfilter.fieldhint" is blacklisted
"avfilter.fieldorder" is blacklisted
"avfilter.find_rect" is blacklisted
"avfilter.floodfill" is blacklisted
"avfilter.fspp" is blacklisted
"avfilter.gblur" is blacklisted
"avfilter.geq" is blacklisted
"avfilter.hflip" is blacklisted
"avfilter.hqdn3d" is blacklisted
"avfilter.hqx" is blacklisted
"avfilter.hue" is blacklisted
"avfilter.hwdownload" is blacklisted
"avfilter.idet" is blacklisted
"avfilter.il" is blacklisted
"avfilter.lenscorrection" is blacklisted
"avfilter.loop" is blacklisted
"avfilter.lumakey" is blacklisted
"avfilter.lut" is blacklisted
"avfilter.lutrgb" is blacklisted
"avfilter.lutyuv" is blacklisted
"avfilter.mcdeint" is blacklisted
"avfilter.metadata" is blacklisted
"avfilter.negate" is blacklisted
"avfilter.nlmeans" is blacklisted
"avfilter.nnedi" is blacklisted
"avfilter.owdenoise" is blacklisted
"avfilter.pad" is blacklisted
"avfilter.perspective" is blacklisted
"avfilter.phase" is blacklisted
"avfilter.pixscope" is blacklisted
"avfilter.pp" is blacklisted
"avfilter.pp7" is blacklisted
"avfilter.prewitt" is blacklisted
"avfilter.realtime" is blacklisted
"avfilter.removegrain" is blacklisted
"avfilter.removelogo" is blacklisted
"avfilter.roberts" is blacklisted
"avfilter.rotate" is blacklisted
"avfilter.scale_vaapi" is blacklisted
"avfilter.showinfo" is blacklisted
"avfilter.shuffleframes" is blacklisted
"avfilter.sidedata" is blacklisted
"avfilter.signalstats" is blacklisted
"avfilter.sobel" is blacklisted
"avfilter.stereo3d" is blacklisted
"avfilter.super2xsai" is blacklisted
"avfilter.swapuv" is blacklisted
"avfilter.tblend" is blacklisted
"avfilter.tlut2" is blacklisted
"avfilter.tonemap" is blacklisted
"avfilter.transpose" is blacklisted
"avfilter.vectorscope" is blacklisted
"avfilter.vflip" is blacklisted
"avfilter.vignette" is blacklisted
"avfilter.vmafmotion" is blacklisted
"avfilter.w3fdif" is blacklisted
"avfilter.xbr" is blacklisted
"avfilter.yadif" is blacklisted
"avfilter.zoompan" is blacklisted
WARNING : Fails to parse  "swresample"
WARNING : Fails to parse  "audiochannels"
WARNING : Fails to parse  "audioconvert"
WARNING : Fails to parse  "data_feed"
"data_show" is blacklisted
WARNING : Fails to parse  "imageconvert"
"luma" is blacklisted
"mask_apply" is blacklisted
"mask_start" is blacklisted
"mono" is blacklisted
"region" is blacklisted
"resize" is blacklisted
"transition" is blacklisted
"watermark" is blacklisted
++++++ Unknown asset :  "avfilter.acompressor"
++++++ Unknown asset :  "avfilter.aecho"
++++++ Unknown asset :  "avfilter.agate"
Warning: duplicate custom definition of effect "frei0r.alpha0ps" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_alpha0ps.xml"
Warning: duplicate custom definition of effect "frei0r.balanc0r" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_balanc0r.xml"
Warning: duplicate custom definition of effect "frei0r.curves" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_curves.xml"
++++++ Unknown asset :  "frei0r.facebl0r"
++++++ Unknown asset :  "frei0r.facedetect"
Warning: duplicate custom definition of effect "frei0r.levels" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_levels.xml"
Warning: duplicate custom definition of effect "frei0r.lightgraffiti" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_lightgraffiti.xml"
Warning: duplicate custom definition of effect "frei0r.select0r" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_select0r.xml"
Warning: duplicate custom definition of effect "frei0r.sopsat" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/frei0r_sopsat.xml"
++++++ Unknown asset :  "frei0r.tehRoxx0r"
++++++ Unknown asset :  "frei0r.vertigo"
++++++ Unknown asset :  "frei0r.vertigo"
++++++ Unknown asset :  "movit.unsharp_mask"
Warning: duplicate custom definition of effect "qtblend" found. Only last one will be considered. Duplicate found in "/usr/share/kdenlive/effects/qtblend.xml"
++++++ Unknown asset :  "region"
++++++ Unknown asset :  "timewarp"
QFSFileEngine::open: No file name specified
"frei0r.alphainjection" is blacklisted
"frei0r.blend" is blacklisted
"frei0r.composition" is blacklisted
"frei0r.xfade0r" is blacklisted
"Could not convert argument 2 at"
	 "expression for index@file:///usr/lib/qt/qml/QtQuick/Controls/Private/TreeViewItemDelegateLoader.qml:84"
	 "insertColumn@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:262"
	 "addColumn@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:236"
	 "expression for onCompleted@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:358"
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"
"Could not convert argument 2 at"
	 "expression for index@file:///usr/lib/qt/qml/QtQuick/Controls/Private/TreeViewItemDelegateLoader.qml:84"
	 "insertColumn@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:262"
	 "addColumn@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:236"
	 "expression for onCompleted@file:///usr/lib/qt/qml/QtQuick/Controls/Private/BasicTableView.qml:358"
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"
qt.qpa.xcb: QXcbConnection: XCB error: 8 (BadMatch), sequence: 1066, resource id: 16777266, major code: 151 (Unknown), minor code: 11
Loading bin playlist...
retain is valid

[...]

Thank you for the backtrace. It seems something with QT version. 
Opened issue: https://invent.kde.org/kde/kdenlive/issues/175

(In reply to emohr from comment #4)
> Thank you for the backtrace. It seems something with QT version. 
> Opened issue: https://invent.kde.org/kde/kdenlive/issues/175

Cool! Hoping you guys can figure it out. The new version of kdenlive seems very promising :-)

Just to doublecheck I downgraded to kdenlive 18, and it did indeed resolve the issue.

In the meanwhile I have acquired a new computer running Intel gpu, and the problem persists. Worse still, after the latest upgrades I am also not able to use kdenlive on xorg/i3wm, as the playback stalls, the open file dialog crashes and so forth. What's happening here?

Hello,

Here is what I posted today on the X2GO mailing list :


"
Hello,

Yesterday, using Kdenlive (absolutely not in the X2GO context), I was bothered by the flickering bug (https://bugs.kde.org/show_bug.cgi?id=406891).
Some comments lead me to try to change my video driver from NVidia to Nouveau xorg driver.

The result is that :
- Kdenlive is not flickering anymore
- Gthumb and VLC are now running nicely through X2GO

This all is beyond my skills, but I figured out a workaround so I thought I'd share it here and there.

Have a nice day.

-- 
Nicolas ECARNOT
"

Thank you for the feedback. Glad to hear it works. 

We close this bug. If it still appears in the latest version, please feel free to re-open it and update the affected version number.

(In reply to emohr from comment #9)
> Thank you for the feedback. Glad to hear it works. 
> 
> We close this bug. If it still appears in the latest version, please feel
> free to re-open it and update the affected version number.

I'm new to kdenlive, and to here. Please bear with me!

System is mint Tara, with onboard intel graphics, and an nvidia card (exact chipsets not to hand ATM).

I've been trying out different versions of kdenlive. The clip monitor works using the (outdated) repository version as well as the later appimage kdenlive-18.12.1b-x86_64.appimage.

However, with kdenlive-19.12.3-x86_64.appimage I get horrendous flickering in the clip window. This occurs when using either graphics hardware.

Stepping through the clip frame by frame (I think! - I'm using the L, R arrow keys) shows occasionally a rendered frame, but mostly a black frame. Sometimes the proper rendered image is shown but is immediately replaced by black. And stepping forwards and backwards gives inconsistent results - sometimes a frame that displayed properly previously becomes black when revisited, and vice versa.

This occurs either when opening an old project, or starting from scratch with a single clip loaded.

I'm using 18.12 for the moment, as 19 is unusable.

(In reply to mike from comment #10)
....
> I'm using 18.12 for the moment, as 19 is unusable.

Checking, I find 19.04.3b fails as does 19.12.3; 8.12.1b works. I've updated the fault report version number to 19.12.3.

Quick update on the wayland situation: it's pretty bad. I am running kdenlive on a Lenovo Thinkpad T480s with Intel GPU, and the flickering has gone even worse than it used to be. When starting from the terminal I get a lot of these kinds of errors:
qrc:/qml/Ruler.qml:121: TypeError: Cannot read property 'timecode' of null
qrc:/qml/Ruler.qml:106: TypeError: Cannot read property 'scaleFactor' of null

and it ends with a:
zsh: segmentation fault (core dumped)  kdenlive

The output of coredumpctl info is:
           PID: 212270 (kdenlive)
           UID: 1000 (kf)
           GID: 1000 (kf)
        Signal: 11 (SEGV)
     Timestamp: Tue 2020-04-21 08:00:24 EEST (2min 16s ago)
  Command Line: kdenlive
    Executable: /usr/bin/kdenlive
 Control Group: /user.slice/user-1000.slice/session-1.scope
          Unit: session-1.scope
         Slice: user-1000.slice
       Session: 1
     Owner UID: 1000 (kf)
       Boot ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    Machine ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
      Hostname: t480s-arch
       Storage: /var/lib/systemd/coredump/core.kdenlive.1000.4d3bb9bc6cf947f6a32eb59485bd8b2e.212270.1587445224000000000000.lz4
       Message: Process 212270 (kdenlive) of user 1000 dumped core.
                
                Stack trace of thread 212270:
                #0  0x0000000000000000 n/a (n/a + 0x0)

System: arch linux 5.6.4-rt3-1-rt, display manager: sway version 1.4
kdenlive version 19.12.3

kdenlive works fine for me in the X environment i3.

(In reply to mike from comment #11)
> (In reply to mike from comment #10)
> ....
> > I'm using 18.12 for the moment, as 19 is unusable.
> 
> Checking, I find 19.04.3b fails as does 19.12.3; 8.12.1b works. I've updated
> the fault report version number to 19.12.3.

Another update.

I'm using the on-board i915 graphics. Testing on another machine with the same chipset, but running Mint Tina, showed no problem.

The problem system has both on-board i915 plus an nvidia card. I blacklisted the nvidia driver and rebooted. This time both 18.x and 19.x crash spectacularly and dump core.

Removing the nvidia blacklist entry, but switching to the nouveau (nv) X11 driver works, albeit 19.x is a bit slow. The monitor display is fine - no flickering.

It looks as though the mere presence of the nvidia driver causes the flickering issue. And even though I'm using the i915 graphics, something to do with the nvidia hardware is affecting things. This is now well beyond my knowledge.

Clip and Project monitor continue to flicker on version 20.08.0 running on ChromeOS. Note that the clip monitor does not flicker when in "Logging" mode. Seems to be related to the fact that the Timeline View is not displayed in this mode.  This problem has been present on ChromeOS since after version 18.12.

Created attachment 130973 [details]
track compositing none

Thanks for the test. Does the flicker go away when you set track compositing to none?

(In reply to emohr from comment #15)
> Created attachment 130973 [details]
> track compositing none
> 
> Thanks for the test. Does the flicker go away when you set track compositing
> to none?

Does not fix. Tried all composting at None, Preview and High Quality and flicker continues to be present.

Here is a scenario to remove the flicker:
1. Start kdenlive and open a clip
2. Go to Editing layout
3. In View menu, disable Time, Effect and Composition (All 3 must be disabled)
> no more flicker in Clip monitor

Do you mean disable following 3 views: Timeline, Effects, and Compositions?

(In reply to emohr from comment #17)
> Do you mean disable following 3 views: Timeline, Effects, and Compositions?

Yes, by disabling those 3 views, the flicker disappears.  Here is a link to a screen recording: https://drive.google.com/file/d/107WNvSeqh443DsOSo3pXOdvnQuAtngsr/view?usp=sharing

(In reply to SergeRGravelle from comment #18)
> (In reply to emohr from comment #17)
> > Do you mean disable following 3 views: Timeline, Effects, and Compositions?
> 
> Yes, by disabling those 3 views, the flicker disappears.  Here is a link to
> a screen recording:
> https://drive.google.com/file/d/107WNvSeqh443DsOSo3pXOdvnQuAtngsr/
> view?usp=sharing

Another bypass that resolves the flickering in the clip and project windows is to  tear-off the clip & project panels into a seperate window.  See video:
https://drive.google.com/file/d/1SvDMIq694B6NypI9fz7hq5w8_PTCF01N/view?usp=sharing

Same as this issue: https://invent.kde.org/multimedia/kdenlive/-/issues/801