Temporary Workaround: KWin Binary Patch

Warning: This is an unofficial workaround that binary patches your compositor (libkwin.so) to skip a Wayland protocol validation check. It makes KWin non-spec-compliant and must be reapplied after every KWin update. Use at your own risk. The proper fix belongs in Proton's Wine Wayland HDR WSI layer, as described above.

If you need HDR working in games right now, this script patches out the min_lum >= max_lum validation in KWin:

#!/bin/bash
# Patch KWin to skip min_lum >= max_lum validation in HDR metadata
# Re-run after every KWin update (pacman -S kwin)

set -euo pipefail

LIBKWIN="/usr/lib/libkwin.so"
BACKUP="${LIBKWIN}.bak"
PATCH_FROM="735f"  # jae (jump if above or equal)
PATCH_TO="9090"    # NOP NOP

find_patch_offset() {
    python3 -c "
import sys

with open('${LIBKWIN}', 'rb') as f:
    data = f.read()

errstr = b'min_lum can\x27t be higher or equal to max_lum'
str_offset = data.find(errstr)
if str_offset == -1:
    print('ERROR: error string not found', file=sys.stderr)
    sys.exit(1)

comisd = b'\x66\x0f\x2f\xc1'
candidates = []
pos = max(0, str_offset - 0x200000)
while True:
    idx = data.find(comisd, pos, str_offset)
    if idx == -1:
        break
    patch_offset = idx + 4
    next_bytes = data[patch_offset:patch_offset + 2]
    if next_bytes == b'\x90\x90':
        candidates.append(patch_offset)
    elif next_bytes[0] == 0x73:
        jae_target = patch_offset + 2 + next_bytes[1]
        if b'\xbe\x05' in data[jae_target:jae_target + 30]:
            candidates.append(patch_offset)
    pos = idx + 1

if not candidates:
    print('ERROR: patch location not found', file=sys.stderr)
    sys.exit(1)

offset = candidates[-1]
print(f'{offset:#x} {data[offset:offset+2].hex()}')
"
}

echo "=== KWin HDR min_lum Patch ==="
RESULT=\$(find_patch_offset)
PATCH_OFFSET=\$(echo "\$RESULT" | awk '{print \$1}')
CURRENT=\$(echo "\$RESULT" | awk '{print \$2}')

echo "Location: \${PATCH_OFFSET}  Current: \${CURRENT}"

if [ "\$CURRENT" = "\$PATCH_TO" ]; then echo "Already patched."; exit 0; fi
if [ "\$CURRENT" != "\$PATCH_FROM" ]; then echo "ERROR: unexpected bytes"; exit 1; fi

[ ! -f "\$BACKUP" ] && sudo cp "\$LIBKWIN" "\$BACKUP"

echo "Patching: \${PATCH_FROM} -> \${PATCH_TO}"
sudo python3 -c "
with open('\${LIBKWIN}', 'r+b') as f:
    f.seek(\${PATCH_OFFSET})
    assert f.read(2) == bytes.fromhex('\${PATCH_FROM}')
    f.seek(\${PATCH_OFFSET})
    f.write(bytes.fromhex('\${PATCH_TO}'))
"
echo "Done. Log out and back in to apply."

What it does: Replaces a jae instruction with two NOPs at the point where KWin validates mastering display luminance values, so it accepts the metadata instead of killing the client.

To revert: sudo cp /usr/lib/libkwin.so.bak /usr/lib/libkwin.so and log out/in. Or just update KWin — pacman overwrites the patch.

Tested on: KDE Plasma 6.6.4, KWin 6.5+, NVIDIA 595.58.03, Crimson Desert with proton-cachyos-slr.

Also reported to KDE as a feature request to consider clamping instead of disconnecting: https://bugs.kde.org/show_bug.cgi?id=519000

Correction: This bug only affects Proton builds that include Wine's native Wayland driver (winewayland.drv), such as proton-cachyos-slr and GE-Proton.

Valve's official Proton Experimental and Hotfix do not ship winewayland.drv — they use XWayland, so they cannot hit this codepath. The original report incorrectly stated it also affects Proton Experimental.

The bug is still relevant for the Proton project since the Wine Wayland driver is upstream and will presumably ship in official Proton builds eventually.