feat: XI2.1 smooth scroll support via xinput driver (#12)

## Summary

Enable pixel-precise smooth scrolling by sending raw pixel deltas
through the `xf86-input-neko` xinput driver instead of quantizing them
into discrete XTest button events.

### Changes

**`server/internal/desktop/xorg.go`** — `Scroll()` now sends raw pixel
deltas directly to the xinput driver via Unix socket. Falls back to
XTest if the driver connection fails.

**`server/internal/webrtc/legacyhandler.go`** — Support
`PayloadScrollWithCtrl` (length=5) binary message format that includes a
`controlKey` byte alongside `deltaX`/`deltaY`.

**`server/pkg/xinput/`** — Add `NEKO_SCROLL` (0x80) message type and
`Scroll(deltaX, deltaY int32)` method to send scroll events over the
Unix socket to the Xorg driver.

**`server/pkg/xorg/xorg.c`** — Fix XTest fallback scroll direction
(button 4/5 mapping was inverted).

**`server/internal/config/desktop.go`** — Add `CDPScrollURL` config
option (unused by default, for optional CDP scroll bypass).

**`server/pkg/cdpscroll/`** — Optional CDP-based scroll client (gated
behind `CDPScrollURL` config, not active by default).

### Context

Used by
[kernel/kernel-images#196](kernel/kernel-images#196)
which extends the `xf86-input-neko` Xorg driver with XI2.1 scroll
valuators. Together, this achieves 1:1 pixel mapping between client
trackpad deltas and browser scroll.

Made with [Cursor](https://cursor.com)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes desktop input/scroll handling (including modifier behavior and
X11 button mapping), which can impact core interaction and has
platform-specific edge cases. Failover paths mitigate risk but need
validation across driver/no-driver setups.
> 
> **Overview**
> Adds XI2.1 *smooth/pixel-precise* scrolling support by routing scroll
deltas through the `xinput` Unix-socket driver when
`desktop.input.enabled` is set, including temporarily setting the Ctrl
modifier before posting the event and falling back to XTest on driver
errors.
> 
> Extends the legacy WebRTC data-channel scroll payload to optionally
include a Ctrl flag (`Length == 5`) while keeping the existing scroll
format for compatibility, and updates logging accordingly.
> 
> Expands `server/pkg/xinput` with a new `NEKO_SCROLL` message and
`Driver.Scroll()` API (dummy returns a not-connected error), and fixes
the XTest fallback scroll direction mapping in `server/pkg/xorg/xorg.c`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ae3594a. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: hiroTamada

server/internal/desktop/manager.go

@@ -37,6 +37,7 @@ type DesktopManagerCtx struct {
 func New(config *config.Desktop) *DesktopManagerCtx {
 	var input xinput.Driver
 	if config.UseInputDriver {
+		log.Info().Str("socket", config.InputSocket).Msg("using xinput driver for scroll")
 		input = xinput.NewDriver(config.InputSocket)
 	} else {
 		input = xinput.NewDummy()

server/internal/desktop/xorg.go

@@ -19,7 +19,21 @@ func (manager *DesktopManagerCtx) GetCursorPosition() (int, int) {
 }
 
 func (manager *DesktopManagerCtx) Scroll(deltaX, deltaY int, controlKey bool) {
-	xorg.Scroll(deltaX, deltaY, controlKey)
+	if manager.config.UseInputDriver {
+		// XI2.1 smooth scrolling via xf86-input-neko: set modifier before the
+		// driver posts the motion event so the X server sees Ctrl held.
+		if controlKey {
+			xorg.SetKeyboardModifier(xorg.KbdModControl, true)
+			defer xorg.SetKeyboardModifier(xorg.KbdModControl, false)
+		}
+		if err := manager.input.Scroll(int32(deltaX), int32(deltaY)); err != nil {
+			manager.logger.Warn().Err(err).Msg("xinput scroll failed, falling back to XTest")
+			xorg.Scroll(deltaX, deltaY, false)
+		}
+	} else {
+		// XTest fallback — handles controlKey atomically under a single X11 lock
+		xorg.Scroll(deltaX, deltaY, controlKey)
+	}
 }
 
 func (manager *DesktopManagerCtx) ButtonDown(code uint32) error {

server/internal/webrtc/legacyhandler.go

@@ -3,7 +3,6 @@ package webrtc
 import (
 	"bytes"
 	"encoding/binary"
-	"strconv"
 
 	"github.com/m1k1o/neko/server/pkg/types"
 
@@ -35,6 +34,13 @@ type PayloadScroll struct {
 	Y int16
 }
 
+type PayloadScrollWithCtrl struct {
+	PayloadHeader
+	DeltaX     int16
+	DeltaY     int16
+	ControlKey uint8
+}
+
 type PayloadKey struct {
 	PayloadHeader
 	Key uint64 // TODO: uint32
@@ -72,18 +78,32 @@ func (manager *WebRTCManagerCtx) handleLegacy(
 
 		manager.desktop.Move(int(payload.X), int(payload.Y))
 	case OP_SCROLL:
-		payload := &PayloadScroll{}
-		if err := binary.Read(buffer, binary.LittleEndian, payload); err != nil {
-			return err
-		}
+		if header.Length == 5 {
+			payload := &PayloadScrollWithCtrl{}
+			if err := binary.Read(buffer, binary.LittleEndian, payload); err != nil {
+				return err
+			}
+
+			logger.Trace().
+				Int16("deltaX", payload.DeltaX).
+				Int16("deltaY", payload.DeltaY).
+				Bool("controlKey", payload.ControlKey != 0).
+				Msg("scroll")
+
+			manager.desktop.Scroll(int(payload.DeltaX), int(payload.DeltaY), payload.ControlKey != 0)
+		} else {
+			payload := &PayloadScroll{}
+			if err := binary.Read(buffer, binary.LittleEndian, payload); err != nil {
+				return err
+			}
 
-		logger.
-			Trace().
-			Str("x", strconv.Itoa(int(payload.X))).
-			Str("y", strconv.Itoa(int(payload.Y))).
-			Msg("scroll")
+			logger.Trace().
+				Int16("x", payload.X).
+				Int16("y", payload.Y).
+				Msg("scroll")
 
-		manager.desktop.Scroll(int(payload.X), int(payload.Y), false)
+			manager.desktop.Scroll(int(payload.X), int(payload.Y), false)
+		}
 	case OP_KEY_DOWN:
 		payload := &PayloadKey{}
 		if err := binary.Read(buffer, binary.LittleEndian, payload); err != nil {

server/internal/webrtc/manager.go

@@ -478,7 +478,6 @@ func (manager *WebRTCManagerCtx) CreatePeer(session types.Session) (*webrtc.Sess
 		//
 
 		if viper.GetBool("legacy") {
-			// handle legacy data channel
 			dc.OnMessage(func(message webrtc.DataChannelMessage) {
 				if err := manager.handleLegacy(logger, message.Data, session); err != nil {
 					logger.Err(err).Msg("data handle failed")

server/pkg/xinput/dummy.go

@@ -1,6 +1,11 @@
 package xinput
 
-import "time"
+import (
+	"errors"
+	"time"
+)
+
+var errNotConnected = errors.New("xinput driver not connected")
 
 type dummy struct{}
 
@@ -29,3 +34,7 @@ func (d *dummy) TouchUpdate(touchId uint32, x, y int, pressure uint8) error {
 func (d *dummy) TouchEnd(touchId uint32, x, y int, pressure uint8) error {
 	return nil
 }
+
+func (d *dummy) Scroll(deltaX, deltaY int32) error {
+	return errNotConnected
+}

server/pkg/xinput/types.go

@@ -12,6 +12,7 @@ const (
 	XI_TouchBegin  = 18
 	XI_TouchUpdate = 19
 	XI_TouchEnd    = 20
+	NEKO_SCROLL    = 0x80
 )
 
 type Message struct {
@@ -58,4 +59,7 @@ type Driver interface {
 	TouchBegin(touchId uint32, x, y int, pressure uint8) error
 	TouchUpdate(touchId uint32, x, y int, pressure uint8) error
 	TouchEnd(touchId uint32, x, y int, pressure uint8) error
+	// scroll via XI2 scroll valuators in the xf86-input-neko driver.
+	// deltaX/deltaY are in scroll units (120 = one notch).
+	Scroll(deltaX, deltaY int32) error
 }

server/pkg/xinput/xinput.go

@@ -120,3 +120,16 @@ func (d *driver) TouchEnd(touchId uint32, x, y int, pressure uint8) error {
 	_, err := d.conn.Write(msg.Pack())
 	return err
 }
+
+func (d *driver) Scroll(deltaX, deltaY int32) error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	msg := Message{
+		_type: NEKO_SCROLL,
+		x:     deltaX,
+		y:     deltaY,
+	}
+	_, err := d.conn.Write(msg.Pack())
+	return err
+}

server/pkg/xorg/xorg.c

@@ -35,16 +35,16 @@ void XScroll(int deltaX, int deltaY) {
 
   int ydir;
   if (deltaY > 0) {
-    ydir = 4; // button 4 is up
+    ydir = 5; // positive = scroll down = button 5
   } else {
-    ydir = 5; // button 5 is down
+    ydir = 4; // negative = scroll up = button 4
   }
 
   int xdir;
   if (deltaX > 0) {
-    xdir = 6; // button 6 is right
+    xdir = 7; // positive = scroll right = button 7
   } else {
-    xdir = 7; // button 7 is left
+    xdir = 6; // negative = scroll left = button 6
   }
 
   for (int i = 0; i < abs(deltaY); i++) {