feat(viewport): optional touchpad pan for wheel-scrolling touchpads

Outside macOS a touchpad's two-finger drag arrives as wheel scrolls that
nothing tells from a wheel. A new option (Options, off by default) makes
every scroll pan and Ctrl+scroll — what a pinch sends there — zoom.

Fixes #1560

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/mod.rs

@@ -559,6 +559,8 @@ pub(super) struct OpenCADStudio {
     polar_increment_deg: f32,
     /// Reverse the mouse-wheel zoom direction when true (ZOOMWHEEL = 1).
     zoom_wheel_reversed: bool,
+    /// Scroll pans and Ctrl+scroll zooms (touchpads); see `scroll_intent`.
+    touchpad_pan: bool,
     /// Mouse-wheel zoom sensitivity (ZOOMFACTOR), held inside
     /// `settings::ZOOM_FACTOR_MIN..=settings::ZOOM_FACTOR_MAX`. `SETVAR`
     /// only sets the range the system variable itself has; the Options
@@ -2375,6 +2377,7 @@ pub enum Message {
     CommandLineFadeChanged(i32),
     /// Toggle reversing the mouse-wheel zoom direction (ZOOMWHEEL).
     ZoomWheelReversedChanged(bool),
+    TouchpadPanChanged(bool),
     /// Change how far one wheel notch zooms, from the Options slider
     /// (ZOOMFACTOR).
     ZoomFactorChanged(i32),
@@ -4157,6 +4160,7 @@ impl OpenCADStudio {
             polar_mode: false,
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
+            touchpad_pan: false,
             zoom_factor: 60,
             zoom_factor_input: 60.to_string(),
             cursor_size: 5,

src/app/settings.rs

@@ -388,6 +388,9 @@ pub struct UserSettings {
     pub polar: bool,
     pub polar_increment_deg: f32,
     pub zoom_wheel_reversed: bool,
+    /// Scrolling pans and Ctrl+scroll (a touchpad pinch) zooms, for a
+    /// touchpad that reports two-finger drags as wheel scrolls.
+    pub touchpad_pan: bool,
     pub zoom_factor: i32,
     /// CURSORSIZE: normalized crosshair reach; 5 retains the original 60 px arms.
     pub cursor_size: i32,
@@ -807,6 +810,7 @@ impl Default for UserSettings {
             polar: false,
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
+            touchpad_pan: false,
             zoom_factor: 60,
             cursor_size: 5,
             pick_box: 3,

src/app/update/file.rs

@@ -662,6 +662,7 @@ impl OpenCADStudio {
             polar: self.polar_mode,
             polar_increment_deg: self.polar_increment_deg,
             zoom_wheel_reversed: self.zoom_wheel_reversed,
+            touchpad_pan: self.touchpad_pan,
             zoom_factor: self.zoom_factor,
             cursor_size: self.cursor_size,
             pick_box: self.pick_box,
@@ -762,6 +763,7 @@ impl OpenCADStudio {
         self.polar_mode = s.polar;
         self.polar_increment_deg = s.polar_increment_deg;
         self.zoom_wheel_reversed = s.zoom_wheel_reversed;
+        self.touchpad_pan = s.touchpad_pan;
         self.zoom_factor = crate::app::settings::clamp_zoom_factor(s.zoom_factor);
         // Like the drafting-rotation field below, the Options field edits a
         // buffer rather than the value, so it is reseeded whenever the value

src/app/update/mod.rs

@@ -8383,6 +8383,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::TouchpadPanChanged(enabled) => {
+                self.touchpad_pan = enabled;
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
             Message::ZoomFactorChanged(factor) => {
                 self.zoom_factor = crate::app::settings::clamp_zoom_factor(factor);
                 // The field beside the slider reads the same value, so the

src/app/update/viewport.rs

@@ -42,9 +42,6 @@ enum ScrollIntent {
     Zoom { notches: f32 },
     /// Trackpad movement to pan by, in screen pixels.
     ///
-    /// Built only where a pixel delta can only mean a trackpad, so it has no
-    /// constructor on the other targets.
-    #[cfg_attr(not(target_os = "macos"), allow(dead_code))]
     Pan { dx: f32, dy: f32 },
 }
 
@@ -55,11 +52,22 @@ const ZOOM_DIR_FLIP_PX: f32 = 2.0;
 
 /// A wheel notch zooms; a trackpad's two fingers pan.
 ///
-/// A wheel reports notches, a precise-scrolling device reports pixels, and that
-/// is the whole distinction — there is no modifier to check and no setting to
-/// read. The pixels only mean "trackpad" on macOS: the browser build reports
-/// every wheel notch as pixels too, so everywhere else they keep zooming.
-fn scroll_intent(delta: mouse::ScrollDelta) -> ScrollIntent {
+/// On macOS a wheel reports notches and a trackpad pixels, and that is the
+/// whole distinction. Elsewhere a touchpad's two-finger drag arrives as wheel
+/// notches (the browser build reports every wheel as pixels), so nothing tells
+/// it from a wheel: with `touchpad_pan` on, every scroll pans and Ctrl+scroll —
+/// what a pinch sends there — zooms.
+fn scroll_intent(delta: mouse::ScrollDelta, touchpad_pan: bool, ctrl: bool) -> ScrollIntent {
+    /// Screen pixels one wheel line pans.
+    const LINE_PX: f32 = 40.0;
+    if touchpad_pan && !ctrl {
+        return match delta {
+            mouse::ScrollDelta::Pixels { x, y } => ScrollIntent::Pan { dx: x, dy: y },
+            mouse::ScrollDelta::Lines { x, y } => {
+                ScrollIntent::Pan { dx: x * LINE_PX, dy: y * LINE_PX }
+            }
+        };
+    }
     match delta {
         // Both axes go straight through: the two-finger gesture moves the
         // drawing the same way the middle-button drag moves it. If a pan ever
@@ -5673,7 +5681,7 @@ properties={:.1}ms picked={}",
     /// A wheel notch zooms; a trackpad's two fingers pan. Which one a delta
     /// means is `scroll_intent`'s call.
     pub(super) fn on_viewport_scroll(&mut self, delta: mouse::ScrollDelta) -> Task<Message> {
-        match scroll_intent(delta) {
+        match scroll_intent(delta, self.touchpad_pan, self.ctrl_down) {
             ScrollIntent::Zoom { notches } => {
                 let mut s =
                     notches * crate::app::settings::zoom_notch_steps(self.zoom_factor);
@@ -6701,10 +6709,24 @@ mod scroll_intent_tests {
     use crate::scene::view::camera::Camera;
     use iced::mouse::ScrollDelta;
 
+    /// With the touchpad option, a scroll pans (wheel lines as pixels) and
+    /// Ctrl+scroll — a pinch — still zooms.
+    #[test]
+    fn touchpad_pan_scrolls_pan_and_ctrl_zooms() {
+        assert_eq!(
+            scroll_intent(ScrollDelta::Lines { x: 0.5, y: -0.25 }, true, false),
+            ScrollIntent::Pan { dx: 20.0, dy: -10.0 }
+        );
+        assert_eq!(
+            scroll_intent(ScrollDelta::Lines { x: 0.0, y: 1.0 }, true, true),
+            ScrollIntent::Zoom { notches: 1.0 }
+        );
+    }
+
     #[test]
     fn wheel_notches_zoom() {
         assert_eq!(
-            scroll_intent(ScrollDelta::Lines { x: 0.0, y: 1.0 }),
+            scroll_intent(ScrollDelta::Lines { x: 0.0, y: 1.0 }, false, false),
             ScrollIntent::Zoom { notches: 1.0 }
         );
     }
@@ -6715,7 +6737,7 @@ mod scroll_intent_tests {
     #[test]
     fn two_finger_scroll_pans() {
         assert_eq!(
-            scroll_intent(ScrollDelta::Pixels { x: 4.0, y: 6.0 }),
+            scroll_intent(ScrollDelta::Pixels { x: 4.0, y: 6.0 }, false, false),
             ScrollIntent::Pan { dx: 4.0, dy: 6.0 }
         );
     }

src/app/view/modal.rs

@@ -316,6 +316,7 @@ impl OpenCADStudio {
                             cliprompt_lines: self.cliprompt_lines,
                             commandline_fade_ms: self.commandline_fade_ms,
                             zoom_wheel_reversed: self.zoom_wheel_reversed,
+                            touchpad_pan: self.touchpad_pan,
                             zoom_factor: self.zoom_factor,
                             texteditmode: self.texteditmode,
                             dimension_continue_mode: self.dimension_continue_mode,

src/locale_catalog.rs

@@ -4797,6 +4797,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "User Preferences" => Some(("common", "user-preferences")),
         "Zoom" => Some(("common", "zoom")),
         "Reverse mouse wheel zoom (ZOOMWHEEL)" => Some(("common", "reverse-mouse-wheel-zoom-zoomwheel")),
+        "Touchpad: scroll pans, Ctrl+scroll or pinch zooms" => Some(("common", "touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms")),
         "Zoom factor" => Some(("common", "zoom-factor")),
         "How far one wheel notch zooms (ZOOMFACTOR)." => Some(("common", "how-far-one-wheel-notch-zooms-zoomfactor")),
         "Text and Dimensions" => Some(("common", "text-and-dimensions")),

src/ui/window/options.rs

@@ -59,6 +59,8 @@ pub struct AppPrefs {
     pub commandline_fade_ms: i32,
     /// ZOOMWHEEL: reverse the mouse-wheel zoom direction.
     pub zoom_wheel_reversed: bool,
+    /// Scroll pans and Ctrl+scroll zooms (touchpads).
+    pub touchpad_pan: bool,
     /// ZOOMFACTOR. The slider sets the range the system variable has; the
     /// field beside it reaches `settings::ZOOM_FACTOR_MAX`.
     pub zoom_factor: i32,
@@ -1254,6 +1256,15 @@ pub fn view_window<'a>(
         ]
         .spacing(8)
         .align_y(iced::Center),
+        Space::new().height(8),
+        row![
+            iced::widget::checkbox(prefs.touchpad_pan)
+                .on_toggle(Message::TouchpadPanChanged)
+                .size(15),
+            text(crate::t!("Touchpad: scroll pans, Ctrl+scroll or pinch zooms")).size(12),
+        ]
+        .spacing(8)
+        .align_y(iced::Center),
         Space::new().height(12),
         row![
             text(crate::t!("Zoom factor")).size(12).width(150),