diff --git a/crates/ui-egui/src/panels/edit.rs b/crates/ui-egui/src/panels/edit.rs
index f423d69e7..7dda3ab54 100644
--- a/crates/ui-egui/src/panels/edit.rs
+++ b/crates/ui-egui/src/panels/edit.rs
@@ -931,8 +931,12 @@ fn draw_curve_histogram(p: &egui::Painter, r: Rect, h: &lightcraft_raster::Histo
 
 /// The four shape controls of the parametric curve, in graph order: region 0 is the darkest.
 pub(crate) const PARAM_AMOUNTS: [&str; 4] = ["curve.shadows", "curve.darks", "curve.lights", "curve.highlights"];
+/// The three region boundaries between those four regions, in graph order.
+pub(crate) const PARAM_SPLITS: [&str; 3] = ["curve.splitShadows", "curve.splitMid", "curve.splitHighlights"];
 /// Resolution the parametric curve is sampled at, matching `lightcraft_pipeline::finish`'s table.
 const PARAM_LUT: usize = 1024;
+/// Radius (screen points) within which the pointer counts as being on a curve handle.
+pub(crate) const CURVE_HIT: f32 = 8.0;
 /// Segments the graph's curve line is drawn with.
 pub(crate) const CURVE_SAMPLES: i32 = 96;
 
@@ -948,13 +952,19 @@ pub(crate) const CURVE_SAMPLES: i32 = 96;
 /// ways can otherwise fold it back on itself, and a fold is exactly what a drawn line used to show
 /// while the renderer straightened it out.
 pub(crate) struct ParametricCurve {
+    /// The curve, sampled and already forced monotone.
     lut: [f32; PARAM_LUT],
+    /// The three region boundaries as fractions of the graph.
+    pub(crate) splits: [f64; 3],
+    /// The four region amounts, in graph order (0 = the darkest).
+    pub(crate) amounts: [f64; 4],
 }
 
 impl ParametricCurve {
     pub(crate) fn new(c: &ToneCurve) -> ParametricCurve {
-        let s = [(c.split_shadows / 100.0) as f32, (c.split_mid / 100.0) as f32, (c.split_highlights / 100.0) as f32];
+        let splits = [c.split_shadows / 100.0, c.split_mid / 100.0, c.split_highlights / 100.0];
         let amounts = [c.shadows, c.darks, c.lights, c.highlights];
+        let s = [splits[0] as f32, splits[1] as f32, splits[2] as f32];
         let regions = [(0.0, s[0], amounts[0]), (s[0], s[1], amounts[1]), (s[1], s[2], amounts[2]), (s[2], 1.0, amounts[3])];
         let mut lut = [0.0_f32; PARAM_LUT];
         let mut prev = 0.0_f32;
@@ -974,7 +984,7 @@ impl ParametricCurve {
             prev = prev.max(y);
             lut[i] = prev;
         }
-        ParametricCurve { lut }
+        ParametricCurve { lut, splits, amounts }
     }
 
     /// The curve at `x` (0 = black, 1 = white), read off the table (so it never overshoots the ends).
@@ -986,6 +996,233 @@ impl ParametricCurve {
         let f = at - i as f32;
         (self.lut[i] + (next - self.lut[i]) * f) as f64
     }
+
+    /// Where region `region`'s handle sits on the input axis: the middle of its own window, so a
+    /// handle never has to cross a split to be grabbed.
+    pub(crate) fn region_x(&self, region: usize) -> f64 {
+        let (a, b) = self.region_window(region);
+        (a + b) / 2.0
+    }
+
+    /// The window (two splits, as fractions of the graph) region `region` owns.
+    fn region_window(&self, region: usize) -> (f64, f64) {
+        let s = [self.splits[0], self.splits[1], self.splits[2]];
+        match region {
+            0 => (0.0, s[0]),
+            1 => (s[0], s[1]),
+            2 => (s[1], s[2]),
+            _ => (s[2], 1.0),
+        }
+    }
+
+    /// The half-width of region `region`'s own bump, as fraction of the graph.
+    fn region_half(&self, region: usize) -> f64 {
+        let (a, b) = self.region_window(region);
+        (b - a) * 0.75 + 0.05
+    }
+
+    /// The cosine window region `region` applies at `x` (1 at its centre, 0 at the window edges).
+    fn region_window_at(&self, region: usize, x: f64) -> f64 {
+        let ctr = self.region_x(region);
+        let t = ((x - ctr) / self.region_half(region)).clamp(-1.0, 1.0);
+        0.5 + 0.5 * (t * std::f64::consts::PI).cos()
+    }
+
+    /// The level region `region`'s bump puts the curve at when its slider holds `amount` — where the
+    /// region's handle is drawn. This is the region's *own* bump (the shape Lightroom draws a handle
+    /// on), not the monotone curve through it: the bump is linear in the amount and one-to-one, so a
+    /// drag inverts it exactly and the handle stays under the pointer.
+    pub(crate) fn handle_level(&self, region: usize, amount: f64) -> f64 {
+        let x = self.region_x(region);
+        let bump = amount / 100.0 * 0.22 * self.region_window_at(region, x) * 4.0 * x * (1.0 - x);
+        (x + bump).clamp(0.0, 1.0)
+    }
+
+    /// The amount that puts region `region`'s handle at `level` — the inverse of [`Self::handle_level`].
+    /// A drag that cannot reach the level asked for saturates at ±100 rather than jumping.
+    pub(crate) fn amount_at_level(&self, region: usize, level: f64) -> f64 {
+        let x = self.region_x(region);
+        let per_100 = 0.22 * self.region_window_at(region, x) * 4.0 * x * (1.0 - x);
+        if per_100.abs() <= f64::EPSILON {
+            return 0.0;
+        }
+        (((level.clamp(0.0, 1.0) - x) / per_100) * 100.0).clamp(-100.0, 100.0).round()
+    }
+
+    /// Where split `split` (0..3) sits on the input axis.
+    pub(crate) fn split_x(&self, split: usize) -> f64 {
+        self.splits.get(split).copied().unwrap_or(0.5)
+    }
+
+    /// The graph position split `split` may be dragged to: inside its own window, a step away from
+    /// the splits either side of it so the four regions keep a width.
+    pub(crate) fn split_x_limited(&self, split: usize, x: f64) -> f64 {
+        let lo = if split == 0 { 0.0 } else { self.split_x(split - 1) + SPLIT_MIN_GAP };
+        let hi = if split + 1 >= PARAM_SPLITS.len() { 1.0 } else { self.split_x(split + 1) - SPLIT_MIN_GAP };
+        if lo > hi { (lo + hi) / 2.0 } else { x.clamp(lo, hi) }
+    }
+
+    /// The value split `split`'s slider takes when its handle is dragged to graph position `x`: the
+    /// position read out in that slider's own range, the way its track reads.
+    pub(crate) fn split_value(&self, split: usize, x: f64) -> f64 {
+        let x = self.split_x_limited(split, x);
+        let id = PARAM_SPLITS.get(split).copied().unwrap_or(PARAM_SPLITS[0]);
+        controls::find(id).map_or(x * 100.0, |s| (s.min + x * (s.max - s.min)).round().clamp(s.min, s.max))
+    }
+
+    /// Screen position of a handle's own centre, the level a drag starts from.
+    fn handle_screen_start(&self, h: ParamHandle, r: Rect) -> Pos2 {
+        let (x, y) = match h {
+            ParamHandle::Region(region) => (self.region_x(region), self.handle_level(region, self.amounts[region])),
+            ParamHandle::Split(split) => (self.split_x(split), 0.0),
+        };
+        pos2(r.left() + x as f32 * r.width(), r.bottom() - y as f32 * r.height())
+    }
+}
+
+/// Close a graph drag's `develop` interaction, as one undo step.
+///
+/// It is closed when the drag ends *or* the pointer comes up without egui reporting `drag_stopped`
+/// for this widget (released outside the window, over a popup, or on a frame the widget does not
+/// run). A dangling interaction would make every later edit a silent preview — Undo would then
+/// appear to do nothing at all. Returns whether the drag is over, so the caller clears its state.
+fn commit_interaction(app: &mut LightcraftApp, ui: &egui::Ui, dragging: bool, stopped: bool) -> bool {
+    let released = ui.input(|i| i.pointer.any_released()) || !ui.input(|i| i.pointer.primary_down());
+    if !dragging || !(stopped || released) {
+        return false;
+    }
+    let _ = app.run("develop.endInteraction", json!({}));
+    true
+}
+
+/// One graph drag of the parametric curve: begin the interaction, write the handle's slider on every
+/// frame of the drag, end it on release. Every write goes through `develop.set`, so the slider under
+/// the graph shows the value the handle holds — and the whole drag is one engine interaction, which
+/// is one undo step (Ctrl+Z takes the gesture back, not the last frame of it).
+fn param_handles(app: &mut LightcraftApp, ui: &mut egui::Ui, r: Rect, resp: &egui::Response, d: &DevelopSettings) {
+    let t = Tokens::get(ui.ctx());
+    let model = ParametricCurve::new(&d.curve);
+    let to_screen = |x: f64, y: f64| pos2(r.left() + x as f32 * r.width(), r.bottom() - y as f32 * r.height());
+    let drag_id = egui::Id::new("curve-param-handle-drag");
+    let mut dragged: Option<ParamHandle> = ui.data(|dd| dd.get_temp::<Option<ParamHandle>>(drag_id)).flatten();
+    let hovered = if dragged.is_none() { resp.hover_pos().and_then(|q| param_handle_at(&model, r, q)) } else { None };
+    let mut set: Option<(&'static str, f64)> = None;
+    // The handle is the one under the *press*, not under where the pointer is once the drag
+    // threshold has passed: that can be most of a handle's own width away, so hit-testing the
+    // current position would grab the wrong control (or none at all on a short, quick drag).
+    let press = ui.input(|i| i.pointer.press_origin());
+    let grab = press.or(resp.interact_pointer_pos());
+    if resp.drag_started()
+        && let Some(h) = grab.and_then(|q| param_handle_at(&model, r, q))
+    {
+        let _ = app.run("develop.beginInteraction", json!({"label": "Parametric Curve"}));
+        dragged = Some(h);
+    }
+    if let Some(h) = dragged
+        && resp.dragged()
+    {
+        // The drag follows how far the pointer has moved, not where it is: the cursor it tracks is
+        // seeded at the handle's own centre (so grabbing a handle never makes its control jump by
+        // the distance between the pointer and that centre) and then advances by the pointer's
+        // motion. ⌥ = a tenth of the motion, as the point curve's drag does.
+        let cursor_id = egui::Id::new("curve-param-cursor");
+        let alt = ui.input(|i| i.modifiers.alt);
+        let dd = ui.input(|i| i.pointer.delta());
+        let step = if alt { vec2(dd.x * 0.1, dd.y * 0.1) } else { vec2(dd.x, dd.y) };
+        let start = model.handle_screen_start(h, r);
+        // seeded at the handle's centre (so the grab itself moves nothing) and advanced by the
+        // pointer's motion from there
+        let advanced = ui.data(|d| d.get_temp::<Pos2>(cursor_id)).unwrap_or(start) + step;
+        ui.data_mut(|d| d.insert_temp(cursor_id, advanced));
+        let level = ((r.bottom() - advanced.y) / r.height()).clamp(0.0, 1.0) as f64;
+        set = Some(match h {
+            ParamHandle::Region(region) => (h.control(), model.amount_at_level(region, level)),
+            ParamHandle::Split(split) => {
+                let x = ((advanced.x - r.left()) / r.width()).clamp(0.0, 1.0) as f64;
+                (h.control(), model.split_value(split, x))
+            }
+        });
+    }
+    if let Some((id, v)) = set {
+        let _ = app.run("develop.set", json!({"control": id, "value": v}));
+    }
+    if commit_interaction(app, ui, dragged.is_some(), resp.drag_stopped()) {
+        dragged = None;
+        // the next drag seeds its cursor from its own handle, not from where this one finished
+        ui.data_mut(|d| d.remove::<Pos2>(egui::Id::new("curve-param-cursor")));
+    }
+    ui.data_mut(|dd| dd.insert_temp(drag_id, dragged));
+    if dragged.is_some() {
+        ui.ctx().set_cursor_icon(egui::CursorIcon::Grabbing);
+    } else if hovered.is_some() {
+        ui.ctx().set_cursor_icon(egui::CursorIcon::Grab);
+    }
+    // the handles: a region's on its own bump, a split's a notch on the bottom edge
+    let p = ui.painter();
+    for region in 0..PARAM_AMOUNTS.len() {
+        let at = to_screen(model.region_x(region), model.handle_level(region, model.amounts[region]));
+        let on = dragged == Some(ParamHandle::Region(region)) || hovered == Some(ParamHandle::Region(region));
+        p.circle_filled(at, if on { 5.5 } else { 4.0 }, Color32::from_gray(230));
+        p.circle_stroke(at, if on { 5.5 } else { 4.0 }, Stroke::new(1.0, t.canvas));
+    }
+    for split in 0..PARAM_SPLITS.len() {
+        let at = to_screen(model.split_x(split), 0.0);
+        let on = dragged == Some(ParamHandle::Split(split)) || hovered == Some(ParamHandle::Split(split));
+        let (w, h) = if on { (2.5, 9.0) } else { (1.5, 6.0) };
+        p.rect_filled(Rect::from_min_max(pos2(at.x - w / 2.0, at.y - h), pos2(at.x + w / 2.0, at.y + h)), 1.0, Color32::from_gray(190));
+    }
+    // what the handle in hand (or under the pointer) holds, as its slider below reads it
+    if let Some(h) = dragged.or(hovered) {
+        let label = controls::get(d, h.control()).map(|v| {
+            let name = controls::find(h.control()).map_or("", |s| s.label);
+            if v > 0.0 { format!("{name} {v:+.0}") } else { format!("{name} {v:.0}") }
+        });
+        if let Some(label) = label {
+            p.text(r.left_top() + vec2(6.0, 4.0), Align2::LEFT_TOP, label, t.font(11.0), t.text_dim);
+        }
+    }
+}
+
+/// Keeps a split clear of its neighbours on the graph.
+const SPLIT_MIN_GAP: f64 = 0.01;
+
+/// A region handle of the parametric graph: which of the four regions, or which of the three splits
+/// between them.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub(crate) enum ParamHandle {
+    /// Region `0..4` (0 = the darkest), grabbed by its slider's amount.
+    Region(usize),
+    /// Split `0..3`, grabbed on the graph's bottom edge.
+    Split(usize),
+}
+
+impl ParamHandle {
+    /// The control this handle writes.
+    pub(crate) fn control(self) -> &'static str {
+        match self {
+            ParamHandle::Region(r) => PARAM_AMOUNTS.get(r).copied().unwrap_or(PARAM_AMOUNTS[0]),
+            ParamHandle::Split(s) => PARAM_SPLITS.get(s).copied().unwrap_or(PARAM_SPLITS[0]),
+        }
+    }
+}
+
+/// The handle under `q`, if any is within [`CURVE_HIT`].
+///
+/// A split's notch is looked for first. It lives on the graph's bottom edge, where the curve itself
+/// passes on the dark end of a shallow line, so on equal terms a region handle would always win and
+/// the notch could never be grabbed.
+fn param_handle_at(model: &ParametricCurve, r: Rect, q: Pos2) -> Option<ParamHandle> {
+    let to_screen = |x: f64, y: f64| pos2(r.left() + x as f32 * r.width(), r.bottom() - y as f32 * r.height());
+    let near = |at: Pos2| at.distance(q) <= CURVE_HIT;
+    for split in 0..PARAM_SPLITS.len() {
+        if near(to_screen(model.split_x(split), 0.0)) {
+            return Some(ParamHandle::Split(split));
+        }
+    }
+    let closest = (0..PARAM_AMOUNTS.len())
+        .map(|region| (region, to_screen(model.region_x(region), model.handle_level(region, model.amounts[region])).distance(q)))
+        .min_by(|a, b| a.1.total_cmp(&b.1))?;
+    (closest.1 <= CURVE_HIT).then_some(ParamHandle::Region(closest.0))
 }
 
 fn curve_editor(app: &mut LightcraftApp, ui: &mut egui::Ui, id: PhotoId, d: &DevelopSettings) {
@@ -1072,6 +1309,7 @@ fn curve_editor(app: &mut LightcraftApp, ui: &mut egui::Ui, id: PhotoId, d: &Dev
         let pts: Vec<Pos2> =
             (0..=CURVE_SAMPLES).map(|i| to_screen(i as f64 / CURVE_SAMPLES as f64, model.at(i as f64 / CURVE_SAMPLES as f64))).collect();
         p.add(egui::Shape::line(pts, Stroke::new(2.0, Color32::from_gray(220))));
+        param_handles(app, ui, r, &resp, d);
         curve_footer(app, ui, d);
         for c in PARAM_AMOUNTS {
             control(app, ui, d, c, true);
@@ -1205,9 +1443,11 @@ fn curve_editor(app: &mut LightcraftApp, ui: &mut egui::Ui, id: PhotoId, d: &Dev
         let arr: Vec<[f64; 2]> = v.iter().map(|p| [p.x, p.y]).collect();
         let _ = app.run("develop.curve", json!({"channel": ch, "points": arr}));
     }
-    if resp.drag_stopped() {
+    // the drag is over when egui says so, or when the pointer comes up without it saying so (a
+    // release outside the window would otherwise leave the interaction open, and every later edit a
+    // silent preview that Undo cannot take back)
+    if commit_interaction(app, ui, dragging.is_some(), resp.drag_stopped()) {
         dragging = None;
-        let _ = app.run("develop.endInteraction", json!({}));
     }
     ui.data_mut(|dd| dd.insert_temp(drag_id, dragging));
     curve_footer(app, ui, d);
diff --git a/crates/ui-egui/src/tests_curve.rs b/crates/ui-egui/src/tests_curve.rs
index 7131c1ade..e918d8d60 100644
--- a/crates/ui-egui/src/tests_curve.rs
+++ b/crates/ui-egui/src/tests_curve.rs
@@ -395,3 +395,176 @@ fn one_graph_drag_that_moves_several_regions_is_one_undo_step() {
     exec(&mut h, "edit.undo", json!({}));
     assert_eq!(develop(&h).curve, before, "undo takes back the whole gesture at once");
 }
+
+/// The screen position of a region's handle: its region's window centre, at the level the region's
+/// own bump puts the curve, so a drag starts on the dot the graph draws.
+fn region_handle(r: [f32; 4], c: &lightcraft_develop::ToneCurve, region: usize) -> [f32; 2] {
+    let model = ParametricCurve::new(c);
+    let x = model.region_x(region);
+    let y = model.handle_level(region, model.amounts[region]);
+    [r[0] + x as f32 * r[2], r[1] + (1.0 - y as f32) * r[3]]
+}
+
+/// The screen position of a split's notch, on the graph's bottom edge.
+fn split_handle(r: [f32; 4], c: &lightcraft_develop::ToneCurve, split: usize) -> [f32; 2] {
+    let model = ParametricCurve::new(c);
+    [r[0] + model.split_x(split) as f32 * r[2], r[1] + r[3]]
+}
+
+/// The point to press to grab a split's notch: its x, a few points up from the graph's bottom edge
+/// (the notch is drawn on the edge, and a press exactly on the widget's border falls outside egui's
+/// hit rect). Well inside [`CURVE_HIT`] of the notch.
+fn split_grab(r: [f32; 4], c: &lightcraft_develop::ToneCurve, split: usize) -> [f32; 2] {
+    let [x, y] = split_handle(r, c, split);
+    [x, y - 3.0]
+}
+
+/// Dragging a region's handle on the graph bends that region and moves its slider with it, live:
+/// the value under the pointer while the drag is in flight, and the same value when it lands.
+#[test]
+fn dragging_a_region_handle_moves_its_slider() {
+    let mut h = curve_open("parametric");
+    exec(&mut h, "develop.set", json!({"control": "curve.lights", "value": 60}));
+    h.step();
+    let r = curve_graph_rect(&mut h);
+    let c = develop(&h).curve;
+    let undo_before = h.app.session.undo.len();
+    let [x, y] = region_handle(r, &c, 2);
+    let down = 0.08 * r[3];
+    let d = h.request("ui.drag", json!({"x": x, "y": y, "toX": x, "toY": y + down, "steps": 8}), T);
+    assert_eq!(d["ok"], true, "{d}");
+    h.step();
+    let after = develop(&h).curve;
+    assert!(after.lights < 60.0, "the Lights slider follows the handle down: {}", after.lights);
+    assert!(after.lights > 0.0, "and lands on a value, not at the bottom: {}", after.lights);
+    assert_eq!((after.shadows, after.darks, after.highlights), (0.0, 0.0, 0.0), "the other regions stay put: {after:?}");
+    assert_eq!(after.split_shadows, c.split_shadows, "and the splits are untouched");
+    assert_eq!(h.app.session.undo.len(), undo_before + 1, "one undo step for the whole drag");
+    // the graph is redrawn bent, with the handle where the drag left it
+    let graph = curve_graph_rect(&mut h);
+    let model = ParametricCurve::new(&after);
+    let painted = painted_param_curve(&h, graph)(model.region_x(2));
+    let want = model.handle_level(2, after.lights);
+    assert!((painted - want).abs() < 0.03, "the handle sits on the redrawn curve: painted {painted}, handle {want}");
+    // and Undo takes the whole gesture back to where it started
+    exec(&mut h, "edit.undo", json!({}));
+    assert_eq!(develop(&h).curve, c, "undo restores the value the drag started from");
+}
+
+/// Dragging up bends the region the other way, so the gesture is not just "downwards works".
+#[test]
+fn dragging_a_region_handle_up_raises_its_slider() {
+    let mut h = curve_open("parametric");
+    let r = curve_graph_rect(&mut h);
+    let c = develop(&h).curve;
+    let [x, y] = region_handle(r, &c, 1);
+    let d = h.request("ui.drag", json!({"x": x, "y": y, "toX": x, "toY": y - 0.08 * r[3], "steps": 8}), T);
+    assert_eq!(d["ok"], true, "{d}");
+    h.step();
+    let after = develop(&h).curve;
+    assert!(after.darks > 0.0, "the Darks slider follows the handle up: {}", after.darks);
+    assert_eq!((after.shadows, after.lights, after.highlights), (0.0, 0.0, 0.0), "the other regions stay put: {after:?}");
+    // the drawn curve really did move up where that region lives
+    assert!(ParametricCurve::new(&after).at(0.375) > ParametricCurve::new(&c).at(0.375), "the curve bends up at the Darks region");
+}
+
+/// Dragging a split's notch moves the region boundary — the same split slider the pipeline reads —
+/// leaving the four region amounts alone, in one undo step.
+#[test]
+fn dragging_a_split_moves_the_region_boundary() {
+    let mut h = curve_open("parametric");
+    exec(&mut h, "develop.set", json!({"control": "curve.lights", "value": 40}));
+    h.step();
+    let r = curve_graph_rect(&mut h);
+    let c = develop(&h).curve;
+    let undo_before = h.app.session.undo.len();
+    let [x, y] = split_grab(r, &c, 1);
+    let d = h.request("ui.drag", json!({"x": x, "y": y, "toX": x + 0.1 * r[2], "toY": y, "steps": 8}), T);
+    assert_eq!(d["ok"], true, "{d}");
+    h.step();
+    let after = develop(&h).curve;
+    assert!(after.split_mid > c.split_mid, "the Midtones split follows the notch right: {} → {}", c.split_mid, after.split_mid);
+    assert_eq!((after.shadows, after.darks, after.lights, after.highlights), (0.0, 0.0, 40.0, 0.0), "the region amounts stay put");
+    assert_eq!(h.app.session.undo.len(), undo_before + 1, "one undo step for the drag");
+    // dragged far right it stops inside the split's own range, without crossing the next split
+    let [x, y] = split_grab(r, &after, 1);
+    let d = h.request("ui.drag", json!({"x": x, "y": y, "toX": x + 0.9 * r[2], "toY": y, "steps": 8}), T);
+    assert_eq!(d["ok"], true, "{d}");
+    h.step();
+    let far = develop(&h).curve;
+    assert!((20.0..=80.0).contains(&far.split_mid), "the split stays in its slider's range: {}", far.split_mid);
+    let model = ParametricCurve::new(&far);
+    assert!(model.split_x(1) < model.split_x(2), "and stays left of the split after it: {far:?}");
+}
+
+/// A drag that starts away from every handle is not a graph gesture at all: nothing moves and no
+/// undo step is made (the graph is not a "click anywhere to bend" surface).
+#[test]
+fn a_parametric_drag_off_a_handle_changes_nothing() {
+    let mut h = curve_open("parametric");
+    exec(&mut h, "develop.set", json!({"control": "curve.lights", "value": 40}));
+    h.step();
+    let before = develop(&h).curve;
+    let undo = h.app.session.undo.len();
+    let r = curve_graph_rect(&mut h);
+    // a corner of the graph, far from the curve and from both handle rows
+    let d = h.request(
+        "ui.drag",
+        json!({"x": r[0] + 0.06 * r[2], "y": r[1] + 0.06 * r[3], "toX": r[0] + 0.2 * r[2], "toY": r[1] + 0.3 * r[3], "steps": 6}),
+        T,
+    );
+    assert_eq!(d["ok"], true, "{d}");
+    h.step();
+    assert_eq!(develop(&h).curve, before, "no control moved");
+    assert_eq!(h.app.session.undo.len(), undo, "and no undo step was made");
+}
+
+/// The graph shows the value under the pointer while the drag is in flight, so the slider and the
+/// handle are seen to agree mid-gesture, not only once it lands.
+#[test]
+fn the_value_under_the_pointer_reads_out_during_the_drag() {
+    let mut h = curve_open("parametric");
+    exec(&mut h, "develop.set", json!({"control": "curve.lights", "value": 60}));
+    h.step();
+    let r = curve_graph_rect(&mut h);
+    let c = develop(&h).curve;
+    let [x, y] = region_handle(r, &c, 2);
+    // press on the handle and hover the same place: the readout names the control it will write
+    let q = h.request("ui.move", json!({"x": x, "y": y}), T);
+    assert_eq!(q["ok"], true, "{q}");
+    h.step();
+    assert!(h.painted_text().iter().any(|t| t.contains("Lights")), "the handle names the slider it writes: {:?}", h.painted_text());
+}
+
+/// A region's handle level and its slider amount are inverses, so a drag puts the handle exactly
+/// where the pointer is; a level the region cannot reach saturates rather than jumping.
+#[test]
+fn a_region_level_and_its_amount_are_inverses() {
+    let c = lightcraft_develop::ToneCurve { shadows: 20.0, lights: 40.0, ..Default::default() };
+    let model = ParametricCurve::new(&c);
+    for region in 0..4 {
+        for amount in [-100.0, -40.0, 0.0, 30.0, 100.0] {
+            let level = model.handle_level(region, amount);
+            let back = model.amount_at_level(region, level);
+            assert!((back - amount).abs() <= 1.0, "region {region}: {amount} → level {level} → {back}");
+        }
+    }
+    assert_eq!(model.amount_at_level(3, 1.0), 100.0, "a level above the region's reach saturates high");
+    assert_eq!(model.amount_at_level(3, 0.0), -100.0, "and one below saturates low");
+    // a split reads out in its own slider's range (10..=70 here), not the graph's 0..=100 …
+    assert_eq!(model.split_value(0, 0.0), 10.0, "the Shadows split bottoms out at its slider's low end");
+    // … and its neighbours are what actually bound it, closer than that range's own ends
+    assert!(model.split_value(0, 1.0) <= 70.0 && model.split_value(0, 1.0) > 10.0, "kept inside its range");
+    assert!(model.split_x(1) > model.split_x(0) && model.split_x(2) > model.split_x(1), "and the splits stay in order");
+    assert!(model.split_value(2, 1.0) <= 90.0, "the Highlights split stays in its own range too");
+}
+
+/// A dragged split never crosses its neighbours, however far the pointer goes.
+#[test]
+fn a_dragged_split_stays_between_its_neighbours() {
+    let model = ParametricCurve::new(&lightcraft_develop::ToneCurve::default());
+    assert!((model.split_x_limited(1, 0.0) - 0.26).abs() < 1e-9, "Midtones stops just right of the split before it");
+    assert!((model.split_x_limited(1, 1.0) - 0.74).abs() < 1e-9, "and just left of the split after it");
+    assert!(model.split_x_limited(0, -1.0).abs() < 1e-9, "the first split can reach black");
+    assert!((model.split_x_limited(2, 2.0) - 1.0).abs() < 1e-9, "the last can reach white");
+}