Some Qt apps not rendering properly in KDE

Hello,

I recently began using KDE Plasma on my system (I've previously used i3 and cinnamon). Although all GTK apps and most Qt apps work as expected, a few Qt apps (such as Virtualbox, Kdenlive, and Qbittorrent) are unusable as they do not render properly. These apps work normally when accessed from i3 or Cinnamon, but only become glitched when launched in KDE. Additionally, I experience some minor screen tearing and compositing issues in Plasma (occasionally windows don't show up until dragged), which could be related.

Screenshot of virtualbox not rendering properly: https://imgur.com/a/opvy8Js

Since issues like this are often caused by nvidia, I have tried the KWIN_TRIPLE_BUFFER solution that is often recommended for similar issues, described on the ArchWiki and in this linked comment, along with disabling Sync to VBank, with no luck:

Any suggestions would be appreciated. Thanks in advance

$ inxi -Fxxxz
System:    Host: manjaro-linux Kernel: 5.4.18-1-MANJARO x86_64 bits: 64 compiler: gcc
           v: 9.2.0 Desktop: KDE Plasma 5.17.5 tk: Qt 5.14.1 wm: kwin_x11 dm: LightDM 1.30.0
           Distro: Manjaro Linux
Machine:   Type: Laptop System: Micro-Star product: GS63 Stealth 8RE v: REV:1.0
           serial: <filter> Chassis: type: 10 serial: <filter>
           Mobo: Micro-Star model: MS-16K5 v: REV:1.0 serial: <filter>
           UEFI: American Megatrends v: E16K5IMS.10F date: 09/04/2018
Battery:   ID-1: BAT1 charge: 57.2 Wh condition: 61.9/65.0 Wh (95%) volts: 12.7/11.4
           model: MSI BIF0_9 type: Li-ion serial: N/A status: Unknown
           Device-1: hidpp_battery_0 model: Logitech Wireless Mouse M325 serial: <filter>
           charge: 55% (should be ignored) rechargeable: yes status: Discharging
CPU:       Topology: 6-Core model: Intel Core i7-8750H bits: 64 type: MT MCP arch: Kaby Lake
           rev: A L2 cache: 9216 KiB
           flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 52815
           Speed: 800 MHz min/max: 800/4100 MHz Core speeds (MHz): 1: 800 2: 800 3: 800
           4: 800 5: 800 6: 800 7: 800 8: 800 9: 800 10: 800 11: 800 12: 800
Graphics:  Device-1: Intel UHD Graphics 630 vendor: Micro-Star MSI driver: i915 v: kernel
           bus ID: 00:02.0 chip ID: 8086:3e9b
           Device-2: NVIDIA GP106M [GeForce GTX 1060 Mobile] vendor: Micro-Star MSI
           driver: nvidia v: 430.64 bus ID: 01:00.0 chip ID: 10de:1c20
           Display: x11 server: X.Org 1.20.7 driver: modesetting,nvidia compositor: kwin_x11
           resolution: 1920x1080~60Hz, 1920x1080~60Hz, 1920x1080~60Hz
           OpenGL: renderer: GeForce GTX 1060/PCIe/SSE2 v: 4.6.0 NVIDIA 430.64
           direct render: Yes
Audio:     Device-1: Intel Cannon Lake PCH cAVS vendor: Micro-Star MSI driver: snd_hda_intel
           v: kernel bus ID: 00:1f.3 chip ID: 8086:a348
           Device-2: NVIDIA GP106 High Definition Audio driver: snd_hda_intel v: kernel
           bus ID: 01:00.1 chip ID: 10de:10f1
           Sound Server: ALSA v: k5.4.18-1-MANJARO
Network:   Device-1: Intel Wireless-AC 9560 [Jefferson Peak] driver: iwlwifi v: kernel
           port: 5000 bus ID: 00:14.3 chip ID: 8086:a370
           IF: wlo1 state: up mac: <filter>
           Device-2: Qualcomm Atheros Killer E2500 Gigabit Ethernet vendor: Micro-Star MSI
           driver: alx v: kernel port: 3000 bus ID: 3b:00.0 chip ID: 1969:e0b1
           IF: enp59s0 state: down mac: <filter>
Drives:    Local Storage: total: 1.14 TiB used: 768.50 GiB (65.7%)
           ID-1: /dev/sda vendor: Western Digital model: WD10SPZX-17Z10T1 size: 931.51 GiB
           speed: 6.0 Gb/s rotation: 5400 rpm serial: <filter> rev: 1A04 scheme: GPT
           ID-2: /dev/sdb vendor: SanDisk model: SD9SN8W256G size: 238.47 GiB
           speed: 6.0 Gb/s serial: <filter> rev: 7000 scheme: GPT
Partition: ID-1: / size: 22.79 GiB used: 20.01 GiB (87.8%) fs: ext4 dev: /dev/sdb6
           ID-2: /home size: 85.43 GiB used: 62.50 GiB (73.2%) fs: ext4 dev: /dev/sdb7
           ID-3: swap-1 size: 7.45 GiB used: 0 KiB (0.0%) fs: swap dev: /dev/sdb5
Sensors:   System Temperatures: cpu: 57.0 C mobo: N/A gpu: nvidia temp: 52 C
           Fan Speeds (RPM): N/A
Info:      Processes: 282 Uptime: 10m Memory: 15.49 GiB used: 2.28 GiB (14.7%) Init: systemd
           v: 242 Compilers: gcc: 9.2.0 Shell: zsh v: 5.7.1 running in: kitty inxi: 3.0.37

After more troubleshooting, the issue appears to be related to the Global Scale setting under Display and Monitor. The glitched UI occurs when set above 1.0x scale, but applications display normally at 1.0x scale. However, if possible I would still like to use a scaling above 1.0x. The solution in this post failed to fix it:

Edit: Partial solution by adding QT_SCREEN_SCALE_FACTORS=1.0 to the .desktop files of broken applications, as recommended here:

Maybe it's a bug in these applications rather than my system...

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