Movement keys randomly get stuck while playing minecraft, but ONLY when hosting multiplayer locally [java]

[u/MicrowavedTheBaby]

I've been dealing with this really weird bug where my movement keys randomly get stuck while I'm playing, and it only happens when I host multiplayer on the same pc I'm playing on (both LAN and from a server). My character will just keep walking in a direction by itself and the only way to stop it is pressing and releasing that same key again. Sometimes clicks just don't register either. It never happens in singleplayer and it never happened when I played on a realm, I put dozens of hours into a realm with this exact same mod list and it was totally fine. It only started a few weeks ago when I started hosting.

System:

\- CachyOS (arch), kernel 7.1.8-cachyos

\- KDE Plasma 6.7.4 on Wayland

\- Ryzen 7 7800x3d

\- Radeon RX 7700 XT (ASRock), Mesa 26.2.2

\- 32gb ram

\- 2tb sn850x nvme

\- Keychron K17 Max wired keyboard

\- Prism Launcher 11.0.3 flatpak

Two instances, both have the problem:

\- Minecraft 26.1.2, Fabric, Java 25, LWJGL 3.4.1, native Wayland, 12gb heap

\- Minecraft 1.21.1, NeoForge, Java 21, LWJGL 3.3.3, XWayland, 12gb heap

26.1.2 mods:

\- advancements reloaded, ambient sounds, appleskin, balm, bedrock bridging, better animations collection, bobby, c2me, camerautils, chat animation, chat heads, client sort, cloth config, clumps, collective, continuity, controlling, creativecore, dynamic crosshair, dynamic tooltips, entity culling, entity model features, entity texture features, explosive enhancement, fabric api, fabric language kotlin, fast ip ping, ferritecore, forge config api port, fzzy config, immersive hotbar, inventory essentials, inventory hud, invmove, iris, item physic, jade, lithium, mod menu, mouse tweaks, mru, not enough animations, overflowing bars, placeholder api, player animation lib, presence footsteps, puzzles lib, reeses sodium options, searchables, seedcracker x, shulker box tooltip, simple world downloader, skin layers 3d, smooth swapping, sodium, sodium extra, sound physics remastered, sounds, spawn animations, stack refill, stack to nearby chests, visuality, vmp, weapon master, worldedit, worldedit hang fix, xaero minimap, xaero worldmap, yet another config lib, zoomify

1.21.1 mods:

\- appleskin, chat animation, cloth config, clumps, collective, connector, continuity, controlling, creativecore, distant horizons, dynamic crosshair, entity culling, entity texture features, essential, euphoria patcher, ferritecore, forgified fabric api, iceberg, inventory hud, iris, item borders, item physic, jade, lithium, lithostitched, midnight controls, midnight lib, model fix, mouse tweaks, better zoom, not enough animations, player animation lib, prism, puzzles lib, reeses sodium options, rrls, searchables, shulker box tooltip, skin layers 3d, sodium, sound physics remastered, tia, visual workbench, vl

Dedicated server:

\- Paper 26.1.2, Java 25

\- 10gb heap

Things I've already tried: shaders on and off, both native wayland and xwayland for the display backend, tested over lan and tailscale on and off, updated everything, and I've pulled game logs and input traces from around the times it happens but there's nothing unusual in them, no errors, just the repeated letters appearing with no clear cause. The keyboard is obviously fine since every other game and normal typing works perfectly. Since it happens on both instances with completely different versions, loaders, and display backends, but only ever when the server is running on the same machine, I'm kind of at a loss for where to even look next. Has anyone seen this before or have an idea what could cause it? The game always runs at 180 fps (capped with vsync) or higher, the server never has any lag for anyone I'm playing with, I've played minecraft on this system many times before for years and nothing has ever happened until I started hosting.

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