I have found another reproducible manifestation of this problem on Zorin
OS 18.1 (Ubuntu 24.04 base).
In my case, OpenRGB 1.0 AppImage crashes during startup with SIGSEGV.
The relevant GDB backtrace is:
```text
#0 ... in ?? () from /lib/x86_64-linux-gnu/libfontconfig.so.1
#1 ... in FcCharSetHasChar()
from /lib/x86_64-linux-gnu/libfontconfig.so.1
#2 ... in QFontEngineMultiFontConfig::shouldLoadFontEngineForCharacter()
from .../libQt6Gui.so.6
#3 ... in QFontEngineMulti::stringToCMap()
#4 ... in QTextEngine::shapeText()
#5 ... in QTextEngine::boundingBox()
...
```
System:
* Zorin OS 18.1
* Ubuntu 24.04 base
* x86_64
* Fontconfig: `2.15.0-1.1ubuntu2`
* OpenRGB 1.0 AppImage
* OpenRGB commit: `81bbe18a84c2e507006f19dd252e397e40a56bfe`
The crash occurs under both Wayland and Xorg.
I also tested a freshly downloaded copy of the same AppImage, so the
problem is not specific to a corrupted OpenRGB download.
On this system, Fontconfig was selecting:
```text
/usr/share/fonts/woff/opendyslexic/OpenDyslexic-Bold.woff
```
for generic font matching.
I found a reproducible workaround by rejecting the OpenDyslexic WOFF
directory through a user-level Fontconfig configuration:
```xml
<selectfont>
<rejectfont>
<glob>/usr/share/fonts/woff/opendyslexic/*</glob>
</rejectfont>
</selectfont>
```
After running `fc-cache -f`, OpenRGB starts normally and `fc-match
"sans-serif"` selects a regular TrueType font instead.
I cannot establish that a single OpenDyslexic WOFF file is the direct
cause, because rejecting individual files did not reproduce the behavior
consistently. However, rejecting the whole OpenDyslexic WOFF set
consistently prevents the crash.
This appears to be another Qt6 application hitting the same
`FcCharSetHasChar()` crash path. I thought this additional case,
especially the reproducible WOFF-font workaround, might be useful for
the investigation.
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https://bugs.launchpad.net/bugs/2168420
Title:
libfontconfig: SIGSEGV in FcCharSetHasChar (rax=0xfefefefefefefefe)
in Qt6 apps with a user font cache in ~/.cache/fontconfig
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