Plasma 6.3.4 may have broken text rendering (or something else) on my system

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Edit:

It’s a Qt 6.9.0 bug. Explanation and workaround here.

Something weird has happened in my Plasma set up: some panes in windows that render text (panes where you can edit text are NOT affected) behave bizarrely, not displaying the text they are supposed to render, and instead displaying glitches—see below. This affects both X11 and Wayland and I have seen this behaviour in KMail, Ghostwriter and Falkon so far.

Here are some photos and screenshots of the issue:

X11

Broken_Ghostwriter_X11
Broken_Ghostwriter_X112686×1518 421 KB

Broken_Kmail_X11
Broken_Kmail_X112690×1518 440 KB

Wayland

PXL_20250406_092919700_1500
PXL_20250406_092919700_15001500×1125 157 KB

PXL_20250406_093002436_1500
PXL_20250406_093002436_15001500×1125 453 KB

PXL_20250406_093305253_1500
PXL_20250406_093305253_15001500×1125 143 KB

This behaviour started after updating to Plasma 6.3.4 last night.

Has anybody else seen anything similar? Is it a new bug or only my set up?

My system’s features are as follows

Operating System: Arch Linux
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.12.0
Qt Version: 6.9.0
Kernel Version: 6.13.8-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 20 × 12th Gen Intel® Core™ i7-12700K
Memory: 62.6 GiB of RAM
Graphics Processor 1: AMD Radeon RX 6700 XT
Graphics Processor 2: zink Vulkan 1.4
Graphics Processor 3: Mesa Intel® UHD Graphics 770
Manufacturer: Micro-Star International Co., Ltd.
Product Name: MS-7D59
System Version: 1.0

A couple of notes:

  • The bug does not affect Firefox, only Falkon.

  • Maybe unrelated: I have also noticed that after a re-start, logging in will not bring up the desktop immediately. Instead I have to press [Enter] three times before the Plasma shell starts loading.