Navigation: hold Space to pan (#1754)

Thanks @styles01.

crates/ui-egui/src/navigation.rs

@@ -387,8 +387,19 @@ pub(crate) fn handle(app: &mut SolveApp, ui: &mut egui::Ui, rect: Rect, resp: &e
     } else if !primary_down && !primary_released {
         app.viewport.primary_nav = None;
     }
-    // The fixed navigation mode (navigation bar) or the preset's left-button binding.
+    // Hold Space for a momentary pan: the hand tool of design apps. It overrides whatever the
-    let primary_nav = app.viewport.nav.or(app.viewport.primary_nav);
+    // left button would otherwise do - selecting, dragging geometry, the preset's binding or the
+    // navigation bar's fixed mode - and lasts only while the key is held. Space still types in a
+    // text field, and while a dimension's text is being edited, so those keep the keyboard.
+    let space_pan = (inside || primary_down)
+        && !ui.ctx().egui_wants_keyboard_input()
+        && !crate::dim_view::editing()
+        && ui.input(|i| i.key_down(egui::Key::Space));
+    if space_pan {
+        ui.ctx().set_cursor_icon(if primary_down { egui::CursorIcon::Grabbing } else { egui::CursorIcon::Grab });
+    }
+    // The fixed navigation mode (navigation bar), a held Space, or the preset's left-button binding.
+    let primary_nav = if space_pan { Some(NavMode::Pan) } else { app.viewport.nav.or(app.viewport.primary_nav) };
     let button = if middle {
         Some(crate::prefs::NavButton::Middle)
     } else if secondary_down {

crates/ui-egui/src/prefs.rs

@@ -1139,7 +1139,7 @@ fn page(app: &mut SolveApp, ui: &mut egui::Ui, t: &Tokens) {
                 note(ui, crate::i18n::tr("nav.touchpad.help"));
             } else {
                 let snap_help = if app.preferences.nav == OPENSCAD_NAV { "" } else { "; hold Shift during orbit to snap" };
-                let raw_help = format!("{}; Ctrl+left drag on empty space orbits{snap_help}; the wheel zooms", nav_preset(&app.preferences.nav).help);
+                let raw_help = format!("{}; Ctrl+left drag on empty space orbits{snap_help}; hold Space to pan; the wheel zooms", nav_preset(&app.preferences.nav).help);
                 note(ui, crate::i18n::tr_label(&raw_help));
             }
             ui.label(crate::i18n::tr_label("Context menu"));

crates/ui-egui/src/viewport.rs

@@ -4111,6 +4111,40 @@ mod tests {
         app
     }
 
+    fn space(pressed: bool) -> egui::Event {
+        egui::Event::Key { key: egui::Key::Space, physical_key: None, pressed, repeat: false, modifiers: egui::Modifiers::default() }
+    }
+
+    /// Holding Space is a momentary pan: the drag moves the view rather than the geometry under the
+    /// pointer, in every preset. The point is over the sketch's own point, so without Space this
+    /// drag would drag it (and the undo stack would grow).
+    #[test]
+    fn holding_space_pans_instead_of_dragging_geometry() {
+        for preset in crate::prefs::NAV_PRESETS.iter().map(|p| p.id) {
+            let ctx = egui::Context::default();
+            let mut app = hover_app(false);
+            app.preferences.nav = preset.into();
+            app.ui.show_origin = false;
+            hover_frame(&mut app, &ctx, vec![]);
+            let point = projection(&app, app.viewport.rect.unwrap()).to_screen(Vec3::ZERO).unwrap();
+            let before = app.cam;
+            let undo = app.session.undo.len();
+            let press = |pressed: bool, at: Pos2| egui::Event::PointerButton {
+                pos: at,
+                button: egui::PointerButton::Primary,
+                pressed,
+                modifiers: egui::Modifiers::default(),
+            };
+            hover_frame(&mut app, &ctx, vec![space(true)]);
+            hover_frame(&mut app, &ctx, vec![space(true), egui::Event::PointerMoved(point), press(true, point)]);
+            hover_frame(&mut app, &ctx, vec![space(true), egui::Event::PointerMoved(point + vec2(40.0, 25.0))]);
+            assert_ne!(app.cam, before, "{preset}: holding Space must pan the view");
+            assert_eq!(app.session.undo.len(), undo, "{preset}: holding Space must not edit the design");
+            assert!(app.viewport.point_drag.is_none(), "{preset}: holding Space must not drag geometry");
+            hover_frame(&mut app, &ctx, vec![press(false, point + vec2(40.0, 25.0)), space(false)]);
+        }
+    }
+
     #[test]
     fn ctrl_left_drag_from_empty_space_orbits_in_every_preset() {
         for preset in crate::prefs::NAV_PRESETS.iter().map(|p| p.id) {