Hi, since today I notice that programs I have opened, and they can be all of them, not one in particular, stick to the panel even though the program is closed again. I can not get rid of them.
I now have my panel filled with everything I had open in the last hour and which is closed as well.

I use Manjaro Linux 16.10.3 KDE version and it is fully updated.

Here is my inxi -Fxzd

System: Host: Desktop_Jan Kernel: 4.4.39-1-MANJARO x86_64 (64 bit gcc: 6.2.1)
Desktop: KDE Plasma 5.8.4 (Qt 5.7.0) Distro: Manjaro Linux
Machine: Device: desktop Mobo: ASUSTeK model: M5A97 R2.0 v: Rev 1.xx BIOS: American Megatrends v: 2601 date: 03/24/2015
CPU: Octa core AMD FX-8350 Eight-Core (-MCP-) cache: 16384 KB
flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm) bmips: 67442
clock speeds: max: 4200 MHz 1: 1400 MHz 2: 2100 MHz 3: 3400 MHz 4: 1400 MHz 5: 3400 MHz 6: 2100 MHz
7: 1400 MHz 8: 1400 MHz
Graphics: Card: NVIDIA GK104 [GeForce GTX 760] bus-ID: 01:00.0
Display Server: X.Org 1.18.4 driver: nvidia Resolution: 1920x1080@60.00hz
GLX Renderer: GeForce GTX 760/PCIe/SSE2 GLX Version: 4.5.0 NVIDIA 375.26 Direct Rendering: Yes
Audio: Card-1 NVIDIA GK104 HDMI Audio Controller driver: snd_hda_intel bus-ID: 01:00.1
Card-2 Advanced Micro Devices [AMD/ATI] SBx00 Azalia (Intel HDA)
driver: snd_hda_intel bus-ID: 00:14.2
Sound: Advanced Linux Sound Architecture v: k4.4.39-1-MANJARO
Network: Card: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller
driver: r8168 v: 8.040.00-NAPI port: d000 bus-ID: 02:00.0
IF: enp2s0 state: up speed: 1000 Mbps duplex: full mac: <filter>
Drives: HDD Total Size: 2256.5GB (40.4% used)
ID-1: /dev/sda model: ADATA_SP900 size: 128.0GB
ID-2: /dev/sdc model: ST1000DM003 size: 1000.2GB
ID-3: /dev/sdb model: ADATA_SP900 size: 128.0GB
ID-4: /dev/sdd model: ST1000DM003 size: 1000.2GB
Optical-1: /dev/sr0 model: ASUS DRW-24F1ST a rev: 1.00 dev-links: cdrom
Features: speed: 48x multisession: yes
audio: yes dvd: yes rw: cd-r,cd-rw,dvd-r,dvd-ram state: running
Optical-2: /dev/sr1 model: ASUS DRW-24F1ST a rev: 1.00 dev-links: N/A
Features: speed: 48x multisession: yes
audio: yes dvd: yes rw: cd-r,cd-rw,dvd-r,dvd-ram state: running
Partition: ID-1: / size: 32G used: 14G (48%) fs: ext4 dev: /dev/sda1
ID-2: /home size: 70G used: 1.6G (3%) fs: ext4 dev: /dev/sda3
ID-3: swap-1 size: 17.18GB used: 0.00GB (0%) fs: swap dev: /dev/sda2
Sensors: System Temperatures: cpu: 27.1C mobo: N/A gpu: 0.0:36C
Fan Speeds (in rpm): cpu: 0
Info: Processes: 235 Uptime: 5:54 Memory: 3685.8/15787.5MB Init: systemd Gcc sys: 6.2.1
Client: Shell (bash 4.4.51) inxi: 2.3.5

I found a thread on a Ubuntu forum as well. The person used KDE4 at the time. Another thread shows a bug report about the same issue in KDE3. Seems, the problem exists much longer already. Are there any more people having this issue?

What can it be, where do I have to look?
Thanks for all your help.

It turns out, in order to avoid the problem I had, with parts of the screen being hardly visible (see thread: https://forum.kde.org/viewtopic.php?f=289&t=138163 I got the tip to use the shift-alt-F12 key-combo to stop the desktop effects. This worked, but caused the problems in this thread.
I managed to solve the other problem as well, so all is well. Thanks.