Add per-axis scroll speed config for input devices (#2109)

* Add per-axis scroll speed config for input devices.

Accepts negative values to inverse scroll direction.

Properly complements/overrides global `scroll-direction` setting.

Includes docs and tests.

* Update per-axis scroll factor implementation after testing

- Refined configuration structure in niri-config
- Updated input handling to use per-axis scroll factors
- Added comprehensive test snapshots
- Updated documentation with per-axis examples

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Simplify per-axis scroll factor implementation per review feedback

- Make documentation concise and clear
- Remove unnecessary comments and test helper functions
- Use inline snapshots for tests
- Rename get_factors() to h_v_factors() for clarity
- Remove unnecessary .clone() calls (ScrollFactor is Copy)
- Reduce test count to essential cases only
- Fix comment about window factor override behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Remove unnecessary ScrollFactor::new() helper function

The maintainer prefers minimal code, so removing this helper and
constructing ScrollFactor directly in tests.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix scroll factor behavior - all settings now multiply with window factor

Per maintainer feedback, both combined and per-axis scroll settings
should multiply with the window-specific scroll factor, not override it.
This ensures consistent behavior regardless of configuration method.

Also removed the now-unused has_per_axis_override() method.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Final cleanup: remove redundant comments and unused snapshot files

- Removed unused snapshot files (now using inline snapshots)
- Removed redundant inline comments in tests
- Simplified test descriptions to be more concise

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert scroll factor parsing tests to use assert_debug_snapshot

Updates parse_scroll_factor_combined, parse_scroll_factor_split, and
parse_scroll_factor_partial tests to use assert_debug_snapshot instead
of manual assert_eq comparisons, as requested in PR review.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert to inline snapshots as requested

- Convert all scroll factor parsing tests to use inline snapshots instead of external files
- Remove external snapshot files to keep test directory clean
- All tests still pass with inline snapshot assertions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix missed assert_eq in parse_scroll_factor_mixed test

Converts the remaining assert_eq calls to assert_debug_snapshot
with inline snapshots in the mixed syntax test function.
Also fixes raw string delimiters from ### to #.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert scroll_factor_h_v_factors test to use assert_debug_snapshot

Makes all scroll factor tests consistent by using snapshots instead
of assert_eq for better maintainability.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fixes

---------

Co-authored-by: Bernardo Kuri <github@bkuri.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

3 files changed, 300 insertions, 21 deletions

diff --git a/docs/wiki/Configuration:-Input.md b/docs/wiki/Configuration:-Input.md
index c9ca0ba2..a03dd67b 100644
--- a/docs/wiki/Configuration:-Input.md
+++ b/docs/wiki/Configuration:-Input.md
@@ -37,6 +37,7 @@ input {
         // accel-speed 0.2
         // accel-profile "flat"
         // scroll-factor 1.0
+        // scroll-factor vertical=1.0 horizontal=-2.0
         // scroll-method "two-finger"
         // scroll-button 273
         // scroll-button-lock
@@ -53,6 +54,7 @@ input {
         // accel-speed 0.2
         // accel-profile "flat"
         // scroll-factor 1.0
+        // scroll-factor vertical=1.0 horizontal=-2.0
         // scroll-method "no-scroll"
         // scroll-button 273
         // scroll-button-lock
@@ -252,6 +254,8 @@ Settings specific to `touchpad` and `mouse`:
 
 - `scroll-factor`: <sup>Since: 0.1.10</sup> scales the scrolling speed by this value.
 
+    <sup>Since: next release</sup> You can also override horizontal and vertical scroll factor separately like so: `scroll-factor horizontal=2.0 vertical=-1.0`
+
 Settings specific to `tablet`s:
 
 - `calibration-matrix`: <sup>Since: 25.02</sup> set to six floating point numbers to change the calibration matrix. See the [`LIBINPUT_CALIBRATION_MATRIX` documentation](https://wayland.freedesktop.org/libinput/doc/latest/device-configuration-via-udev.html) for examples.
diff --git a/niri-config/src/lib.rs b/niri-config/src/lib.rs
index 16ec6dca..dbd6b5ea 100644
--- a/niri-config/src/lib.rs
+++ b/niri-config/src/lib.rs
@@ -191,6 +191,25 @@ pub enum TrackLayout {
     Window,
 }
 
+#[derive(knuffel::Decode, Debug, Default, Clone, Copy, PartialEq)]
+pub struct ScrollFactor {
+    #[knuffel(argument)]
+    pub base: Option<FloatOrInt<0, 100>>,
+    #[knuffel(property)]
+    pub horizontal: Option<FloatOrInt<-100, 100>>,
+    #[knuffel(property)]
+    pub vertical: Option<FloatOrInt<-100, 100>>,
+}
+
+impl ScrollFactor {
+    pub fn h_v_factors(&self) -> (f64, f64) {
+        let base_value = self.base.map(|f| f.0).unwrap_or(1.0);
+        let h = self.horizontal.map(|f| f.0).unwrap_or(base_value);
+        let v = self.vertical.map(|f| f.0).unwrap_or(base_value);
+        (h, v)
+    }
+}
+
 #[derive(knuffel::Decode, Debug, Default, PartialEq)]
 pub struct Touchpad {
     #[knuffel(child)]
@@ -227,8 +246,8 @@ pub struct Touchpad {
     pub disabled_on_external_mouse: bool,
     #[knuffel(child)]
     pub middle_emulation: bool,
-    #[knuffel(child, unwrap(argument))]
-    pub scroll_factor: Option<FloatOrInt<0, 100>>,
+    #[knuffel(child)]
+    pub scroll_factor: Option<ScrollFactor>,
 }
 
 #[derive(knuffel::Decode, Debug, Default, PartialEq)]
@@ -251,8 +270,8 @@ pub struct Mouse {
     pub left_handed: bool,
     #[knuffel(child)]
     pub middle_emulation: bool,
-    #[knuffel(child, unwrap(argument))]
-    pub scroll_factor: Option<FloatOrInt<0, 100>>,
+    #[knuffel(child)]
+    pub scroll_factor: Option<ScrollFactor>,
 }
 
 #[derive(knuffel::Decode, Debug, Default, PartialEq)]
@@ -4055,6 +4074,237 @@ mod tests {
     }
 
     #[test]
+    fn parse_scroll_factor_combined() {
+        // Test combined scroll-factor syntax
+        let parsed = do_parse(
+            r#"
+            input {
+                mouse {
+                    scroll-factor 2.0
+                }
+                touchpad {
+                    scroll-factor 1.5
+                }
+            }
+            "#,
+        );
+
+        assert_debug_snapshot!(parsed.input.mouse.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: Some(
+                    FloatOrInt(
+                        2.0,
+                    ),
+                ),
+                horizontal: None,
+                vertical: None,
+            },
+        )
+        "#);
+        assert_debug_snapshot!(parsed.input.touchpad.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: Some(
+                    FloatOrInt(
+                        1.5,
+                    ),
+                ),
+                horizontal: None,
+                vertical: None,
+            },
+        )
+        "#);
+    }
+
+    #[test]
+    fn parse_scroll_factor_split() {
+        // Test split horizontal/vertical syntax
+        let parsed = do_parse(
+            r#"
+            input {
+                mouse {
+                    scroll-factor horizontal=2.0 vertical=-1.0
+                }
+                touchpad {
+                    scroll-factor horizontal=-1.5 vertical=0.5
+                }
+            }
+            "#,
+        );
+
+        assert_debug_snapshot!(parsed.input.mouse.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: None,
+                horizontal: Some(
+                    FloatOrInt(
+                        2.0,
+                    ),
+                ),
+                vertical: Some(
+                    FloatOrInt(
+                        -1.0,
+                    ),
+                ),
+            },
+        )
+        "#);
+        assert_debug_snapshot!(parsed.input.touchpad.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: None,
+                horizontal: Some(
+                    FloatOrInt(
+                        -1.5,
+                    ),
+                ),
+                vertical: Some(
+                    FloatOrInt(
+                        0.5,
+                    ),
+                ),
+            },
+        )
+        "#);
+    }
+
+    #[test]
+    fn parse_scroll_factor_partial() {
+        // Test partial specification (only one axis)
+        let parsed = do_parse(
+            r#"
+            input {
+                mouse {
+                    scroll-factor horizontal=2.0
+                }
+                touchpad {
+                    scroll-factor vertical=-1.5
+                }
+            }
+            "#,
+        );
+
+        assert_debug_snapshot!(parsed.input.mouse.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: None,
+                horizontal: Some(
+                    FloatOrInt(
+                        2.0,
+                    ),
+                ),
+                vertical: None,
+            },
+        )
+        "#);
+        assert_debug_snapshot!(parsed.input.touchpad.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: None,
+                horizontal: None,
+                vertical: Some(
+                    FloatOrInt(
+                        -1.5,
+                    ),
+                ),
+            },
+        )
+        "#);
+    }
+
+    #[test]
+    fn parse_scroll_factor_mixed() {
+        // Test mixed base + override syntax
+        let parsed = do_parse(
+            r#"
+            input {
+                mouse {
+                    scroll-factor 2 vertical=-1
+                }
+                touchpad {
+                    scroll-factor 1.5 horizontal=3
+                }
+            }
+            "#,
+        );
+
+        assert_debug_snapshot!(parsed.input.mouse.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: Some(
+                    FloatOrInt(
+                        2.0,
+                    ),
+                ),
+                horizontal: None,
+                vertical: Some(
+                    FloatOrInt(
+                        -1.0,
+                    ),
+                ),
+            },
+        )
+        "#);
+        assert_debug_snapshot!(parsed.input.touchpad.scroll_factor, @r#"
+        Some(
+            ScrollFactor {
+                base: Some(
+                    FloatOrInt(
+                        1.5,
+                    ),
+                ),
+                horizontal: Some(
+                    FloatOrInt(
+                        3.0,
+                    ),
+                ),
+                vertical: None,
+            },
+        )
+        "#);
+    }
+
+    #[test]
+    fn scroll_factor_h_v_factors() {
+        let sf = ScrollFactor {
+            base: Some(FloatOrInt(2.0)),
+            horizontal: None,
+            vertical: None,
+        };
+        assert_debug_snapshot!(sf.h_v_factors(), @r#"
+        (
+            2.0,
+            2.0,
+        )
+        "#);
+
+        let sf = ScrollFactor {
+            base: None,
+            horizontal: Some(FloatOrInt(3.0)),
+            vertical: Some(FloatOrInt(-1.0)),
+        };
+        assert_debug_snapshot!(sf.h_v_factors(), @r#"
+        (
+            3.0,
+            -1.0,
+        )
+        "#);
+
+        let sf = ScrollFactor {
+            base: Some(FloatOrInt(2.0)),
+            horizontal: Some(FloatOrInt(1.0)),
+            vertical: None,
+        };
+        assert_debug_snapshot!(sf.h_v_factors(), @r"
+        (
+            1.0,
+            2.0,
+        )
+        ");
+    }
+
+    #[test]
     fn parse() {
         let parsed = do_parse(
             r##"
@@ -4370,9 +4620,15 @@ mod tests {
                     disabled_on_external_mouse: true,
                     middle_emulation: false,
                     scroll_factor: Some(
-                        FloatOrInt(
-                            0.9,
-                        ),
+                        ScrollFactor {
+                            base: Some(
+                                FloatOrInt(
+                                    0.9,
+                                ),
+                            ),
+                            horizontal: None,
+                            vertical: None,
+                        },
                     ),
                 },
                 mouse: Mouse {
@@ -4394,9 +4650,15 @@ mod tests {
                     left_handed: false,
                     middle_emulation: true,
                     scroll_factor: Some(
-                        FloatOrInt(
-                            0.2,
-                        ),
+                        ScrollFactor {
+                            base: Some(
+                                FloatOrInt(
+                                    0.2,
+                                ),
+                            ),
+                            horizontal: None,
+                            vertical: None,
+                        },
                     ),
                 },
                 trackpoint: Trackpoint {
diff --git a/src/input/mod.rs b/src/input/mod.rs
index c2667dfc..cc07aed3 100644
--- a/src/input/mod.rs
+++ b/src/input/mod.rs
@@ -3086,31 +3086,44 @@ impl State {
 
         self.update_pointer_contents();
 
-        let scroll_factor = match source {
-            AxisSource::Wheel => self.niri.config.borrow().input.mouse.scroll_factor,
-            AxisSource::Finger => self.niri.config.borrow().input.touchpad.scroll_factor,
-            _ => None,
+        let device_scroll_factor = {
+            let config = self.niri.config.borrow();
+            match source {
+                AxisSource::Wheel => config.input.mouse.scroll_factor,
+                AxisSource::Finger => config.input.touchpad.scroll_factor,
+                _ => None,
+            }
         };
-        let scroll_factor = scroll_factor.map(|x| x.0).unwrap_or(1.);
 
+        // Get window-specific scroll factor
         let window_scroll_factor = pointer
             .current_focus()
             .map(|focused| self.niri.find_root_shell_surface(&focused))
             .and_then(|root| self.niri.layout.find_window_and_output(&root).unzip().0)
-            .and_then(|window| window.rules().scroll_factor);
-        let scroll_factor = scroll_factor * window_scroll_factor.unwrap_or(1.);
+            .and_then(|window| window.rules().scroll_factor)
+            .unwrap_or(1.);
+
+        // Determine final scroll factors based on configuration
+        let (horizontal_factor, vertical_factor) = device_scroll_factor
+            .map(|x| x.h_v_factors())
+            .unwrap_or((1.0, 1.0));
+        let (horizontal_factor, vertical_factor) = (
+            horizontal_factor * window_scroll_factor,
+            vertical_factor * window_scroll_factor,
+        );
 
         let horizontal_amount = horizontal_amount.unwrap_or_else(|| {
             // Winit backend, discrete scrolling.
             horizontal_amount_v120.unwrap_or(0.0) / 120. * 15.
-        }) * scroll_factor;
+        }) * horizontal_factor;
+
         let vertical_amount = vertical_amount.unwrap_or_else(|| {
             // Winit backend, discrete scrolling.
             vertical_amount_v120.unwrap_or(0.0) / 120. * 15.
-        }) * scroll_factor;
+        }) * vertical_factor;
 
-        let horizontal_amount_v120 = horizontal_amount_v120.map(|x| x * scroll_factor);
-        let vertical_amount_v120 = vertical_amount_v120.map(|x| x * scroll_factor);
+        let horizontal_amount_v120 = horizontal_amount_v120.map(|x| x * horizontal_factor);
+        let vertical_amount_v120 = vertical_amount_v120.map(|x| x * vertical_factor);
 
         let mut frame = AxisFrame::new(event.time_msec()).source(source);
         if horizontal_amount != 0.0 {