When using X11 forwarding from non-standard X servers (e.g. XQuartz,
Xephyr), the XKB key names reported by the remote server do not match
the expected evdev names, causing all keys to map to RDP_SCANCODE_UNKNOWN.
Add a keysym-to-scancode lookup table as fallback. Unlike XKB key names,
keysyms are standardized across X11 implementations. The fallback only
fills keycodes that remain unmapped after the XKB pass, so standard
setups are unaffected.
Tested with X11 forwarding from Exegol (Docker) via XQuartz on macOS,
and natively on Linux and macOS.
Related to #4215
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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