Weird behavior with stylus on Linux

[CleyFaye]

Hi,

I'm facing a weird issue, and I'm not even sure how I could debug this.
I'm using Weylus on a Linux PC with X, with the browser being chrome on a lenovo Android tablet.

The version I run was built an hour ago, from the GIT repository, using the provided docker container, and installed as a .deb.

I have properly configured uinput, and most of the time, things works extremely smoothly: low latency video on the tablet, almost immediate stylus action on the PC, pressure work, position is correct, window/screen selection works fine, etc.

But SOMETIMES, when approaching the stylus to the tablet's screen, a set of spurious input will be thrown on the PC.
On Krita, this results in the layers being ALL flattened, the de-saturate effect being applied, and the grid showing up. This would translate to something like ctrl+shift+E, ctrl+shift+U, ctrl+shift+' being sent in quick sequence, or something similar.

A similarly weird behavior can be seen, sometimes, when using photopea on the PC, where it will de-saturate dozens of times.

Unfortunately, I'm not sure if the PC gets all these inputs or something else. I tried running xev, but it segfault immediately:

MappingNotify event, serial 40, synthetic NO, window 0x0,
    request MappingKeyboard, first_keycode 8, count 248
^]^]^\Quitter (core dumped)

Weylus itself doesn't give out much details:

2025-11-21T01:21:06.379522Z  INFO weylus::gui: Found more than one IP address for browsers to connect to,
2025-11-21T01:21:06.379540Z  INFO weylus::gui: other urls are:
2025-11-21T01:21:06.379544Z  INFO weylus::gui: http://10.8.0.34:1701
2025-11-21T01:21:06.379550Z  INFO weylus::gui: http://10.11.42.7:1701
2025-11-21T01:21:06.379554Z  INFO weylus::gui: http://172.17.0.1:1701
2025-11-21T01:21:06.379559Z  INFO weylus::gui: http://172.18.0.1:1701
2025-11-21T01:21:20.473477Z  INFO weylus::log: Video: 1920x540@h264_nvenc pix_fmt: cuda (bgr0)
2025-11-21T01:21:26.105082Z  INFO weylus::log: Video: 780x290@h264_nvenc pix_fmt: cuda (bgr0)
2025-11-21T01:22:17.537343Z  INFO weylus::log: Video: 1920x1080@h264_nvenc pix_fmt: cuda (bgr0)
XRequest.130: BadAccess (attempt to access private resource denied) 0x8a00004
XRequest.130: BadShmSeg (invalid shared segment parameter) 0x8a00005
2025-11-21T01:27:16.844524Z  INFO weylus::web: Webserver is shutting down.
XRequest.131: XI_BadDevice (invalid Device parameter) 0x17

I have no idea what's happening. And after this happens, the program works fine for a while, before doing that again, seemingly at random.
If I disable uinput, the issue goes away, but so does the pressure (obviously). But I'm not sure what else I could do.

Is there anything obvious I'm missing, or something else I can try to investigate? I'm not familiar at all with Rust or its tools, so I'm not too sure. If there's an easy way to build the deb in debug mode, I could start with that.

[H-M-H]

Thanks for the report, you can do some more debugging by running Weylus with extra debug logging like this: WEYLUS_LOG_LEVEL=TRACE weylus.

[CleyFaye]

Ah, thanks for the pointer.

The tl;dr:
Firefox on Android will sometimes send weird keydown/keyup with no code value and "Unidentified" as the key.
Although I have no idea why, this is somewhat "documented" on MDN regarding keyboard event code (in the range 0x00DA~0x01CF).
I did a dirty patch before building the webapp to ignore any event with "Unidentifier" as key, and it seems to help.

The long version:

Here's the log I got (I removed most of the library loading, client connecting, resolution changing, as it seems to not be really relevant):

TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE Loaded lib: libnvidia-encode.so.1
TRACE Loaded sym: NvEncodeAPICreateInstance
TRACE Loaded sym: NvEncodeAPIGetMaxSupportedVersion
DEBUG Loaded Nvenc version 13.0
DEBUG Nvenc initialized successfully
DEBUG CQ(5376) mode enabled.
DEBUG Empty MOOV enabled; disabling automatic bitstream filtering
INFO Video: 1762x992@h264_nvenc pix_fmt: cuda (bgr0)
DEBUG Nvenc unloaded
DEBUG video frame properties congruent with link at pts_time: NOPTS
TRACE Dropped 26 frame(s)!
TRACE Received message: Config(ClientConfiguration { uinput_support: true, capturable_id: 1, capture_cursor: false, max_width: 1920, max_height: 1073, client_name: None, frame_rate: 29.977193414327505 })
TRACE Received message: Config(ClientConfiguration { uinput_support: true, capturable_id: 1, capture_cursor: false, max_width: 1920, max_height: 1152, client_name: None, frame_rate: 29.977193414327505 })
TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE CUDA texture alignment: 512
TRACE Setting 'video_size' to value '1920x1080'
TRACE Setting 'pix_fmt' to value '121'
TRACE Setting 'time_base' to value '1/1000'
TRACE Setting 'pixel_aspect' to value '1/1'
DEBUG w:1920 h:1080 pixfmt:bgr0 tb:1/1000 fr:0/1 sar:1/1 csp:unknown range:unknown
TRACE Setting 'w' to value '1920'
TRACE Setting 'h' to value '1080'
TRACE Setting 'format' to value 'bgr0'
TRACE Setting 'interp_algo' to value 'nearest'
TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE query_formats: 4 queried, 9 merged, 0 already done, 0 delayed
TRACE CUDA texture alignment: 512
DEBUG w:1920 h:1080 fmt:bgr0 -> w:1920 h:1080 fmt:bgr0 (passthrough)
DEBUG Scale filter set bgr0 -> cuda (sw: bgr0) up!
TRACE Setting 'video_size' to value '1920x1080'
TRACE Setting 'pix_fmt' to value '119'
TRACE Setting 'time_base' to value '1/1000'
TRACE Setting 'pixel_aspect' to value '1/1'
DEBUG w:1920 h:1080 pixfmt:rgb0 tb:1/1000 fr:0/1 sar:1/1 csp:unknown range:unknown
TRACE Setting 'w' to value '1920'
TRACE Setting 'h' to value '1080'
TRACE Setting 'format' to value 'bgr0'
TRACE Setting 'interp_algo' to value 'nearest'
TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE query_formats: 4 queried, 9 merged, 0 already done, 0 delayed
TRACE CUDA texture alignment: 512
TRACE CUDA texture alignment: 512
DEBUG w:1920 h:1080 fmt:rgb0 -> w:1920 h:1080 fmt:bgr0
DEBUG Scale filter set rgb0 -> cuda (sw: bgr0) up!
TRACE Setting 'video_size' to value '1920x1080'
TRACE Setting 'pix_fmt' to value '2'
TRACE Setting 'time_base' to value '1/1000'
TRACE Setting 'pixel_aspect' to value '1/1'
DEBUG w:1920 h:1080 pixfmt:rgb24 tb:1/1000 fr:0/1 sar:1/1 csp:unknown range:unknown
TRACE Setting 'w' to value '1920'
TRACE Setting 'h' to value '1080'
TRACE Setting 'flags' to value 'fast_bilinear'
DEBUG w:1920 h:1080 flags:'fast_bilinear' interl:0
TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE query_formats: 4 queried, 9 merged, 0 already done, 0 delayed
TRACE picking rgb0 out of 9 ref:rgb24 alpha:0
DEBUG w:1920 h:1080 fmt:rgb24 csp:gbr range:pc sar:1/1 -> w:1920 h:1080 fmt:rgb0 csp:gbr range:pc sar:1/1 flags:0x00000001
DEBUG Selected 1/1000 time base
DEBUG Sync level 1
TRACE CUDA texture alignment: 512
DEBUG Scale filter set rgb24 -> cuda (sw: bgr0) up!
TRACE Loaded lib: libcuda.so.1
// Bunch of Loaded sym
TRACE Loaded lib: libnvidia-encode.so.1
TRACE Loaded sym: NvEncodeAPICreateInstance
TRACE Loaded sym: NvEncodeAPIGetMaxSupportedVersion
DEBUG Loaded Nvenc version 13.0
DEBUG Nvenc initialized successfully
DEBUG CQ(5376) mode enabled.
DEBUG Empty MOOV enabled; disabling automatic bitstream filtering
INFO Video: 1920x1080@h264_nvenc pix_fmt: cuda (bgr0)
DEBUG Nvenc unloaded
DEBUG video frame properties congruent with link at pts_time: NOPTS
TRACE Dropped 24 frame(s)!
//
// Client close the panel in the browser, then stylus approached the display, didn't touch it
//
TRACE Received message: PointerEvent(PointerEvent { event_type: OVER, pointer_id: 0, timestamp: 18379000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(PRIMARY), x: 0.6899999789428717, y: 0.04488889058430989, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: ENTER, pointer_id: 0, timestamp: 18379000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(PRIMARY), x: 0.6899999789428717, y: 0.04488889058430989, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: DOWN, pointer_id: 0, timestamp: 18370000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(PRIMARY), x: 0.6899999789428717, y: 0.04488889058430989, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 18449000, is_primary: true, pointer_type: Touch, button: Button(0x0), buttons: Button(PRIMARY), x: 0.7184999780731208, y: -0.02355555534362793, movement_x: -76, movement_y: 1, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 18453000, is_primary: true, pointer_type: Touch, button: Button(0x0), buttons: Button(PRIMARY), x: 0.7234999931793215, y: -0.03333333333333333, movement_x: -69, movement_y: -6, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: UP, pointer_id: 0, timestamp: 18462000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(0x0), x: 0.7234999931793215, y: -0.03333333333333333, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: OUT, pointer_id: 0, timestamp: 18467000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(0x0), x: 0.7234999931793215, y: -0.03333333333333333, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: PointerEvent(PointerEvent { event_type: LEAVE, pointer_id: 0, timestamp: 18469000, is_primary: true, pointer_type: Touch, button: Button(PRIMARY), buttons: Button(0x0), x: 0.7234999931793215, y: -0.03333333333333333, movement_x: 0, movement_y: 0, pressure: 0.0010000000474974513, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0, height: 0.0 })
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknown key:  code: Unidentified, trying to insert unicode using ctrl + shift + u + 556E6964656E746966696564!
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknow key: code:  key: Unidentified, ignoring event.
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknown key:  code: Unidentified, trying to insert unicode using ctrl + shift + u + 556E6964656E746966696564!
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknow key: code:  key: Unidentified, ignoring event.
TRACE Received message: PointerEvent(PointerEvent { event_type: OVER, pointer_id: 0, timestamp: 22645000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43066666666666664, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: ENTER, pointer_id: 0, timestamp: 22646000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43066666666666664, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22618000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43066666666666664, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22793000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.4368888753255208, movement_x: 0, movement_y: 5, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22804000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43777779134114586, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22815000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43777779134114586, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22826000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.43866666666666665, movement_x: 0, movement_y: 1, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22837000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.4395555419921875, movement_x: 0, movement_y: 1, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22848000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.4404444580078125, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22859000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.44133333333333336, movement_x: 0, movement_y: 1, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22870000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.44222220865885414, movement_x: 0, movement_y: 1, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22882000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.44222220865885414, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 22889000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.4124999874114994, y: 0.44222220865885414, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 })

It seems that the server do receive some unknown key event (I assure you I'm nowhere near a keyboard, virtual or not, on the android tablet).
These key event are "inserted" as ctrl+shift+u+556E6964656E746966696564

Now, "556E6964656E746966696564" is actually "Unidentified", so I gather from that that weylus is receiving something, and trying to actually type "ctrl+shift+u+U+n+i+d+e+n+t+i+f+i+e+d", but I'm not sure it actually happens.
I can, however, confirm that the weird behavior is not specific to Krita.

Since xev kept crashing, I tried something simpler, to no avail:

$ cat > data
^]^]^^^E^\^\Quitter (core dumped)
$ cat data
$ ls -lh data
-rw-rw-r-- 1 cleyfaye cleyfaye 0 nov.  23 18:22 data

The above happened when I approached the stylus to the screen.
Note that it does not happen all the time.
I had to move the stylus closer, than away, three times.
The first two times there's only PointerEvent in the log (and no weird behavior on the computer).

The keyboard events isolated:

TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknown key:  code: Unidentified, trying to insert unicode using ctrl + shift + u + 556E6964656E746966696564!
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknow key: code:  key: Unidentified, ignoring event.
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknown key:  code: Unidentified, trying to insert unicode using ctrl + shift + u + 556E6964656E746966696564!
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknow key: code:  key: Unidentified, ignoring event.
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknown key:  code: Unidentified, trying to insert unicode using ctrl + shift + u + 556E6964656E746966696564!
TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
DEBUG Got unknow key: code:  key: Unidentified, ignoring event.
TRACE Received message: PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 142335000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.10714091125027853, y: 0.7608800998261295, movement_x: 0, movement_y: 0, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0007116877421532723, height: 0.0007116877421532723 })

So, I tested with different things enabled:

• with only "mouse", I get proper mouse movement, and lits of PointerEvent(PointerEvent { event_type: MOVE, pointer_id: 0, timestamp: 38674000, is_primary: true, pointer_type: Mouse, button: Button(0x0), buttons: Button(0x0), x: 0.11349999272155784, y: 0.6413333333333333, movement_x: 0, movement_y: 3, pressure: 0.0, tilt_x: 0, tilt_y: 0, twist: 0, width: 0.0006536816376894422, height: 0.0006536816376894422 }) (which seems normal)
• with only "stylus" enabled, I did get spurious keyboard inputs:

  TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: DOWN, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })
  TRACE Received message: KeyboardEvent(KeyboardEvent { event_type: UP, code: "", key: "Unidentified", location: STANDARD, alt: false, ctrl: false, shift: false, meta: false })`
  

At this point, either the browser (up-to-date Firefox on Android) is sending bogus inputs that are somehow passed through by Weylus, or something weird(er) is going on.

I wrote a simple event logger to check from within Firefox (only keydown/keyup/input), and sure enough, I managed to get those keystroke:

KEYDOWN {"code":"","altKey":false,"ctrlKey":false,"key":"Unidentified","location":0,"metaKey":false,"shiftKey":false} logger.html:13:11
  {"isTrusted":true} logger.html:15:11
  keydown { target: div#panel
  , key: "Unidentified", charCode: 0, keyCode: 0 }
  altKey: false
  bubbles: true
  cancelBubble: false
  cancelable: true
  charCode: 0
  code: ""
  composed: true
  ctrlKey: false
  currentTarget: null
  defaultPrevented: false
  detail: 0
  eventPhase: 0
  explicitOriginalTarget: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  isComposing: false
  isTrusted: true
  key: "Unidentified"
  keyCode: 0
  layerX: 0
  layerY: 0
  location: 0
  metaKey: false
  originalTarget: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  rangeOffset: 0
  rangeParent: null
  repeat: false
  returnValue: true
  shiftKey: false
  srcElement: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">​
  target: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  timeStamp: 53219
  type: "keydown"
  view: Window http://192.168.34.167:8000/logger.html
  which: 0
  <get isTrusted()>: function isTrusted()
  <prototype>: KeyboardEventPrototype { getModifierState: getModifierState(), initKeyboardEvent: initKeyboardEvent(), charCode: Getter, … }
  logger.html:16:11
KEYUP {"code":"","altKey":false,"ctrlKey":false,"key":"Unidentified","location":0,"metaKey":false,"shiftKey":false} logger.html:13:11
  {"isTrusted":true} logger.html:15:11
  keyup { target: div#panel
  , key: "Unidentified", charCode: 0, keyCode: 0 }
  altKey: false
  bubbles: true
  cancelBubble: false
  cancelable: true
  charCode: 0
  code: ""
  composed: true
  ctrlKey: false
  currentTarget: null
  defaultPrevented: false
  detail: 0
  eventPhase: 0
  explicitOriginalTarget: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  isComposing: false
  isTrusted: true
  key: "Unidentified"
  keyCode: 0
  layerX: 0
  layerY: 0
  location: 0
  metaKey: false
  originalTarget: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  rangeOffset: 0
  rangeParent: null
  repeat: false
  returnValue: true
  shiftKey: false
  srcElement: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">​
  target: <div id="panel" tabindex="0" style="width: 100vw; height: 10…p: 0; overflow: scroll;">
  timeStamp: 53219
  type: "keyup"
  view: Window http://192.168.34.167:8000/logger.html
  which: 0

They happen rarely, but they're there.
There is no input event with them (my snippet do try to catch those).

Since these do not seem to do anything, and are fired randomly (some people ran into a similar issue, at least according to https://clark.engineering/input-on-android-229-unidentified-1d92105b9a04?gi=196958b6c861 ), I just hotfixed the code to skip event where key === 'Unidentified and rebuilt the .deb.

I lack the knowledge needed to know if this will have unintended side effects, but as far as I can tell, it did fix the immediate issue of extremely weird input sequence coming through weylus.

The fix in question, in ts/lib.ts:868:

        d.onkeydown = (e) => {
            if (!settings_hidden())
                return true;
            if (e.key === "Unidentified") return true;
            if (e.repeat)
                return this.onEvent(e, "repeat");
            return this.onEvent(e, "down");
        };   
        d.onkeyup = (e) => {
            if (!settings_hidden())
                return true;
            if (e.key === "Unidentified") return true;
            return this.onEvent(e, "up");
        };   
        d.onkeypress = (e) => {
            if (!settings_hidden())
                return true;
            if (e.key === "Unidentified") return true;
            e.preventDefault();
            e.stopPropagation();
            return false;
        };   

It's dirty, but maybe this would make sense, even if behind a flag.
With this, I was unable to trigger the weird behavior, and everything else, as far as I know, works smoothly (mouse movement when hovering, stylus pressure, etc.).