path: root/packages/ext-stage-dock/tests/test_policy.cpp

#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include <doctest/doctest.h>

#include "anim.hpp"
#include "dock_layout.hpp"
#include "gesture.hpp"
#include "reveal.hpp"

// Pure-core tests — the heart of this b4 step. No kernel, no wlroots, no RMLUi.
// Four cores: the reveal recognizer (reversible edge swipe -> fraction +
// commit), the dock layout geometry (reveal fraction -> rects), the e1 gesture
// Controller (touch/drag STREAM -> slide_px/dragging/open + the Outcome the glue
// applies), and the d1-fix SlideAnimator (the interruptible easing animator that
// drives slide_px over time for the keyboard/minimize/restore/drag-release
// paths). All four need nothing running.

namespace rv = unbox::ext_stage_dock::reveal;
namespace lay = unbox::ext_stage_dock::layout;
namespace gst = unbox::ext_stage_dock::gesture;
namespace anm = unbox::ext_stage_dock::anim;

using rv::RevealCommit;
using rv::RevealConfig;
using rv::RevealRecognizer;

// ============================================================================
// reveal recognizer
// ============================================================================

// A config with round numbers so fractions are exact: 100px dock, threshold
// 0.5, fling 1.0 px/ms, 24px edge slop.
static auto cfg() -> RevealConfig {
    return RevealConfig{.dock_width = 100, .open_threshold = 0.5, .fling_velocity = 1.0, .edge_slop = 24};
}

TEST_CASE("begin: an edge-started press is a reveal; a non-edge press is not") {
    RevealRecognizer r(cfg());
    CHECK(r.begin(0.0, 200.0, 0));      // exactly the left edge
    RevealRecognizer r2(cfg());
    CHECK(r2.begin(24.0, 200.0, 0));    // exactly at edge_slop
    RevealRecognizer r3(cfg());
    CHECK_FALSE(r3.begin(25.0, 200.0, 0));  // just past the slop -> not a reveal
    RevealRecognizer r4(cfg());
    CHECK_FALSE(r4.begin(500.0, 200.0, 0)); // mid-screen -> not a reveal
}

TEST_CASE("fraction grows with inward drag and is clamped to [0,1]") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    CHECK(r.fraction() == doctest::Approx(0.0));
    CHECK(r.update(50.0, 0.0, 100) == doctest::Approx(0.5)); // 50/100 dock_width
    CHECK(r.update(100.0, 0.0, 200) == doctest::Approx(1.0)); // full
    CHECK(r.update(180.0, 0.0, 300) == doctest::Approx(1.0)); // clamped at 1
}

TEST_CASE("fraction shrinks on reverse drag (reversible) and clamps at 0") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    CHECK(r.update(80.0, 0.0, 100) == doctest::Approx(0.8));
    CHECK(r.update(30.0, 0.0, 200) == doctest::Approx(0.3)); // dragged back inward->edge
    CHECK(r.update(0.0, 0.0, 300) == doctest::Approx(0.0));  // back to the edge
    CHECK(r.update(-40.0, 0.0, 400) == doctest::Approx(0.0)); // past the edge -> clamp 0
}

TEST_CASE("slow drag past threshold commits open; below threshold commits close") {
    RevealRecognizer open(cfg());
    REQUIRE(open.begin(0.0, 0.0, 0));
    open.update(60.0, 0.0, 1000);   // fraction 0.6, 60px over 1000ms -> 0.06 px/ms (slow)
    CHECK(open.end(1000) == RevealCommit::open);

    RevealRecognizer close(cfg());
    REQUIRE(close.begin(0.0, 0.0, 0));
    close.update(40.0, 0.0, 1000);  // fraction 0.4 (slow) -> below threshold
    CHECK(close.end(1000) == RevealCommit::close);
}

TEST_CASE("at-threshold (==open_threshold) commits open") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    r.update(50.0, 0.0, 1000); // exactly 0.5
    CHECK(r.end(1000) == RevealCommit::open);
}

TEST_CASE("a fast inward fling under threshold still commits open (velocity)") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    // Only 30px (fraction 0.3, below 0.5) but in 10ms -> 3 px/ms >= 1.0 fling.
    r.update(30.0, 0.0, 10);
    CHECK(r.fraction() == doctest::Approx(0.3));
    CHECK(r.end(10) == RevealCommit::open);
}

TEST_CASE("a fast outward fling over threshold commits close (velocity)") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    r.update(90.0, 0.0, 1000);    // fraction 0.9 (well past threshold), slow
    r.update(60.0, 0.0, 1005);    // yanked back 30px in 5ms -> -6 px/ms outward fling
    CHECK(r.fraction() == doctest::Approx(0.6)); // still past threshold by position
    CHECK(r.end(1005) == RevealCommit::close);   // but the fling closes it
}

TEST_CASE("symmetric CLOSE drag: seed start_fraction=1.0, drag toward edge to close") {
    // The already-open dock: begin at the dock's right edge with start_fraction
    // 1.0, then drag back toward the screen edge. The SAME recognizer drives it.
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0, /*start_fraction=*/1.0));
    CHECK(r.fraction() == doctest::Approx(1.0));
    // Drag inward->edge by 70px slowly: 1.0 + (-70/100) = 0.3, below threshold.
    r.update(-70.0, 0.0, 1000);
    CHECK(r.fraction() == doctest::Approx(0.3));
    CHECK(r.end(1000) == RevealCommit::close);
}

TEST_CASE("symmetric CLOSE drag that does not travel far stays open") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0, /*start_fraction=*/1.0));
    r.update(-20.0, 0.0, 1000); // 1.0 - 0.2 = 0.8, still past threshold, slow
    CHECK(r.end(1000) == RevealCommit::open);
}

TEST_CASE("dt==0 samples do not divide by zero and keep prior velocity") {
    RevealRecognizer r(cfg());
    REQUIRE(r.begin(0.0, 0.0, 0));
    r.update(30.0, 0.0, 10);          // 3 px/ms inward
    const double v = r.velocity();
    r.update(40.0, 0.0, 10);          // same timestamp -> velocity unchanged
    CHECK(r.velocity() == doctest::Approx(v));
    CHECK(r.fraction() == doctest::Approx(0.4)); // fraction still tracks position
}

TEST_CASE("an inactive (non-edge) recognizer is inert") {
    RevealRecognizer r(cfg());
    CHECK_FALSE(r.begin(500.0, 0.0, 0));
    CHECK_FALSE(r.active());
    CHECK(r.update(600.0, 0.0, 100) == doctest::Approx(0.0)); // no movement
    CHECK(r.end(100) == RevealCommit::close);
}

// ============================================================================
// dock layout
// ============================================================================

static auto metrics() -> lay::DockMetrics {
    return lay::DockMetrics{.output_w = 1920, .output_h = 1080, .dock_width = 300};
}

TEST_CASE("dock_box: f=0 fully off-screen left, f=1 flush at x==0") {
    auto m = metrics();
    auto hidden = lay::dock_box(m, 0.0);
    CHECK(hidden.x == -300); // -dock_width
    CHECK(hidden.w == 300);
    CHECK(hidden.y == 0);
    CHECK(hidden.h == 1080); // covers the output

    auto shown = lay::dock_box(m, 1.0);
    CHECK(shown.x == 0);
    CHECK(shown.w == 300);
    CHECK(shown.h == 1080);
}

TEST_CASE("dock_box: x is monotonic non-decreasing in f and clamps outside [0,1]") {
    auto m = metrics();
    CHECK(lay::dock_box(m, 0.5).x == -150); // halfway
    int prev = lay::dock_box(m, 0.0).x;
    for (double f = 0.0; f <= 1.0; f += 0.1) {
        int x = lay::dock_box(m, f).x;
        CHECK(x >= prev);
        prev = x;
    }
    CHECK(lay::dock_box(m, -1.0).x == -300); // clamped to f=0
    CHECK(lay::dock_box(m, 2.0).x == 0);     // clamped to f=1
}

// FULL-HEIGHT RAIL: the dock surface is kDockWidth wide x the FULL OUTPUT HEIGHT
// tall, REGARDLESS of card count (the RCSS owns the in-rail flex centering +
// overflow scroll; the C++ no longer sizes the surface to the card stack). The
// revealed frame the glue feeds to UiSurfaceSpec/set_position is dock_box(m, 1.0)
// at x==0, full output height — this is what the glue's create_dock_surface uses
// for spec.width/height. (The earlier hug-the-cards content_height/surface_height
// helpers were removed with the rail change; the RCSS scrolls the overflow.)
TEST_CASE("dock_box: revealed rail is dock_width x full output height, count-independent") {
    // Various output heights -> the rail's h always equals output_h (never the
    // card-stack content height); w always dock_width; revealed x == 0.
    for (int oh : {600, 1080, 1440}) {
        lay::DockMetrics m{.output_w = 1920, .output_h = oh, .dock_width = 288};
        auto rail = lay::dock_box(m, 1.0);
        CHECK(rail.x == 0);
        CHECK(rail.y == 0);
        CHECK(rail.w == 288);
        CHECK(rail.h == oh); // FULL output height, independent of any card count
    }
}

// ============================================================================
// gesture Controller (e1) — the touch/drag STREAM -> state transition
// ============================================================================

using gst::Controller;

// Matches the recognizer test config: 100px dock so fractions are exact, 0.5
// threshold, 1.0 px/ms fling, 24px edge slop. metrics: 100px dock_width so
// dock_box(f).x = -100*(1-f) (closed = -100, open = 0).
static auto ctrl() -> Controller {
    return Controller(
        RevealConfig{.dock_width = 100, .open_threshold = 0.5, .fling_velocity = 1.0,
                     .edge_slop = 24},
        lay::DockMetrics{.output_w = 1920, .output_h = 1080, .dock_width = 100});
}

TEST_CASE("Controller: full OPEN stream past 50% ends open, flush, visible") {
    Controller c = ctrl();
    CHECK(c.open() == false);
    CHECK(c.dragging() == false);

    // touch_down at the very edge: begins the OPEN reveal.
    auto down = c.touch_down(/*id=*/1, /*lx=*/0.0, /*ly=*/200.0, /*t=*/0);
    CHECK(down.make_visible);
    CHECK(down.dirty_slide);
    CHECK(down.dirty_dragging);
    CHECK(c.dragging());
    CHECK(c.gesturing());
    CHECK(c.slide_px() == doctest::Approx(-100.0)); // f=0 fully hidden

    // motion inward to 60px -> fraction 0.6 -> slide -40.
    auto m1 = c.touch_motion(1, 60.0, 200.0, 1000);
    CHECK(m1.dirty_slide);
    CHECK_FALSE(m1.dirty_dragging);
    CHECK(c.slide_px() == doctest::Approx(-40.0));

    // release at 0.6 (>= 0.5, slow) -> OPEN, slide 0, surface visible.
    auto up = c.touch_up(1, 1000);
    CHECK(up.make_visible);
    CHECK(up.dirty_slide);
    CHECK(up.dirty_dragging);
    CHECK(c.open());
    CHECK_FALSE(c.dragging());
    CHECK_FALSE(c.gesturing());
    CHECK(c.slide_px() == doctest::Approx(0.0));
}

TEST_CASE("Controller: slide_px advances monotonically with inward motion") {
    Controller c = ctrl();
    REQUIRE(c.touch_down(1, 0.0, 0.0, 0).make_visible);
    double prev = c.slide_px();
    for (double x : {10.0, 30.0, 55.0, 80.0, 100.0}) {
        c.touch_motion(1, x, 0.0, static_cast<std::uint32_t>(x) + 100);
        CHECK(c.slide_px() >= prev);
        prev = c.slide_px();
    }
    CHECK(c.slide_px() == doctest::Approx(0.0)); // f=1 flush
}

TEST_CASE("Controller: OPEN release below 50% ends closed") {
    Controller c = ctrl();
    REQUIRE(c.touch_down(1, 0.0, 0.0, 0).make_visible);
    c.touch_motion(1, 40.0, 0.0, 1000); // fraction 0.4, slow
    auto up = c.touch_up(1, 1000);
    CHECK(up.dirty_slide);
    CHECK(up.dirty_dragging);
    CHECK_FALSE(up.make_visible); // close: no make_visible (dock_settled hides)
    CHECK_FALSE(c.open());
    CHECK_FALSE(c.dragging());
    CHECK(c.slide_px() == doctest::Approx(-100.0)); // closed offset
}

TEST_CASE("Controller: fast inward fling below 50% still opens") {
    Controller c = ctrl();
    REQUIRE(c.touch_down(1, 0.0, 0.0, 0).make_visible);
    c.touch_motion(1, 30.0, 0.0, 10); // 0.3 fraction but 3 px/ms >= fling
    auto up = c.touch_up(1, 10);
    CHECK(up.make_visible);
    CHECK(c.open());
    CHECK(c.slide_px() == doctest::Approx(0.0));
}

TEST_CASE("Controller: edge-slop rejection — a press past the slop is ignored") {
    Controller c = ctrl();
    auto down = c.touch_down(1, /*lx=*/25.0, 0.0, 0); // just past 24px slop
    CHECK_FALSE(down.make_visible);
    CHECK_FALSE(down.dirty_slide);
    CHECK_FALSE(down.dirty_dragging);
    CHECK_FALSE(c.gesturing());
    CHECK_FALSE(c.dragging());
    // Subsequent motion/up for that id are no-ops (no active gesture).
    auto m = c.touch_motion(1, 80.0, 0.0, 100);
    CHECK_FALSE(m.dirty_slide);
    auto up = c.touch_up(1, 100);
    CHECK_FALSE(up.dirty_slide);
    CHECK_FALSE(c.open());
}

TEST_CASE("Controller: a touch_down while already open is ignored") {
    Controller c = ctrl();
    c.open_now();
    REQUIRE(c.open());
    auto down = c.touch_down(1, 0.0, 0.0, 0); // edge press, but dock is open
    CHECK_FALSE(down.make_visible);
    CHECK_FALSE(down.dirty_slide);
    CHECK_FALSE(c.gesturing());
    CHECK(c.open()); // unchanged
}

TEST_CASE("Controller: motion/up for a non-active touch id are ignored") {
    Controller c = ctrl();
    REQUIRE(c.touch_down(1, 0.0, 0.0, 0).make_visible);
    auto m = c.touch_motion(/*other id=*/2, 80.0, 0.0, 100);
    CHECK_FALSE(m.dirty_slide);
    CHECK(c.slide_px() == doctest::Approx(-100.0)); // unchanged by the foreign id
    auto up = c.touch_up(2, 100);
    CHECK_FALSE(up.dirty_slide);
    CHECK(c.gesturing()); // id 1 still active
}

TEST_CASE("Controller: touch_cancel reverts the active OPEN gesture to closed") {
    Controller c = ctrl();
    REQUIRE(c.touch_down(1, 0.0, 0.0, 0).make_visible);
    c.touch_motion(1, 90.0, 0.0, 1000); // dragged well open (0.9)
    auto cancel = c.touch_cancel(1);
    CHECK(cancel.dirty_dragging);
    CHECK(cancel.dirty_slide);
    CHECK_FALSE(cancel.make_visible);
    CHECK_FALSE(c.open());
    CHECK_FALSE(c.dragging());
    CHECK_FALSE(c.gesturing());
    CHECK(c.slide_px() == doctest::Approx(-100.0));
}

TEST_CASE("Controller: CLOSE drag toward the edge below 50% closes") {
    Controller c = ctrl();
    c.open_now();
    REQUIRE(c.open());
    CHECK(c.slide_px() == doctest::Approx(0.0));

    // drag_start force-active at fraction 1.0 (finger lands anywhere on the open
    // dock). dragging on, no slide change yet.
    auto start = c.drag_start(/*x=*/50.0, /*y=*/200.0, /*t=*/0);
    CHECK(start.dirty_dragging);
    CHECK(c.dragging());

    // drag_move back toward the edge: from x=50 to x=-20 is -70px travel ->
    // 1.0 + (-70/100) = 0.3 fraction -> slide -70.
    auto move = c.drag_move(-20.0, 200.0, 1000);
    CHECK(move.dirty_slide);
    CHECK(c.slide_px() == doctest::Approx(-70.0));

    // drag_end at 0.3 (< 0.5, slow) -> CLOSE.
    auto end = c.drag_end(1000);
    CHECK(end.dirty_dragging);
    CHECK(end.dirty_slide);
    CHECK_FALSE(end.make_visible);
    CHECK_FALSE(c.open());
    CHECK_FALSE(c.dragging());
    CHECK(c.slide_px() == doctest::Approx(-100.0));
}

TEST_CASE("Controller: a CLOSE drag that barely travels stays open") {
    Controller c = ctrl();
    c.open_now();
    c.drag_start(50.0, 0.0, 0);
    c.drag_move(30.0, 0.0, 1000); // -20px -> 0.8, still past threshold, slow
    auto end = c.drag_end(1000);
    CHECK(c.open());
    CHECK_FALSE(c.dragging());
    CHECK(c.slide_px() == doctest::Approx(0.0)); // snapped back to open
    CHECK(end.dirty_slide);
}

TEST_CASE("Controller: open_now / close_now set the target + Outcome flags") {
    Controller c = ctrl();
    auto o = c.open_now();
    CHECK(o.make_visible);
    CHECK(o.dirty_slide);
    CHECK(o.dirty_dragging);
    CHECK(c.open());
    CHECK_FALSE(c.dragging());
    CHECK(c.slide_px() == doctest::Approx(0.0));

    auto cl = c.close_now();
    CHECK_FALSE(cl.make_visible);
    CHECK(cl.dirty_slide);
    CHECK(cl.dirty_dragging);
    CHECK_FALSE(c.open());
    CHECK(c.slide_px() == doctest::Approx(-100.0));
}

TEST_CASE("Controller: set_metrics re-scales the slide offset for a new output") {
    Controller c = ctrl();
    c.set_metrics(lay::DockMetrics{.output_w = 1920, .output_h = 1080, .dock_width = 100});
    c.close_now();
    CHECK(c.slide_px() == doctest::Approx(-100.0));
    // dock_box only uses dock_width for .x, so changing only output_h keeps it.
    c.set_metrics(lay::DockMetrics{.output_w = 2560, .output_h = 1440, .dock_width = 100});
    c.close_now();
    CHECK(c.slide_px() == doctest::Approx(-100.0));
}

// ============================================================================
// SlideAnimator (d1 fix) — the interruptible easing animator over time
// ============================================================================

using anm::SlideAnimator;

// A linear tween (identity) so lerped values are exact at any progress.
static auto linear() -> std::function<float(float)> {
    return [](float t) { return t; };
}

TEST_CASE("SlideAnimator: starts inactive and reports its initial value") {
    SlideAnimator a;
    CHECK_FALSE(a.active());
    CHECK(a.value() == doctest::Approx(0.0));
    // tick while inactive is inert (returns the held value, does not advance).
    CHECK(a.tick(0.016) == doctest::Approx(0.0));
    CHECK_FALSE(a.active());
}

TEST_CASE("SlideAnimator: linear run progresses monotonically and ends exactly at target") {
    // A 0.5s run; ticks of 0.05s (well under the kMaxTickDt clamp) so the
    // accumulated elapsed is exact and the loop terminates deterministically.
    SlideAnimator a;
    a.start(/*from=*/-100.0, /*to=*/0.0, /*duration=*/0.5, linear());
    CHECK(a.active());
    CHECK(a.value() == doctest::Approx(-100.0)); // at start: from

    // Advance in 0.05s steps; value must never go backward and tracks the lerp.
    double prev = a.value();
    for (int i = 0; i < 9; ++i) {
        double v = a.tick(0.05);
        CHECK(v >= prev); // monotonic for an inward (increasing) run
        prev = v;
        CHECK(a.active()); // still mid-run before the final step (0.45s < 0.5s)
    }
    // 0.45s elapsed -> 90% of the way: -100 + 90 = -10.
    CHECK(a.value() == doctest::Approx(-10.0));

    // The step that crosses duration lands EXACTLY on `to` and goes inactive. A
    // final dt of 0.1 (the clamp) clears the remaining ~0.05s unambiguously.
    double end = a.tick(0.1); // elapsed > duration -> done
    CHECK(end == doctest::Approx(0.0)); // exactly the target, no float drift
    CHECK_FALSE(a.active());
    // Further ticks are inert and stay pinned at the target.
    CHECK(a.tick(0.5) == doctest::Approx(0.0));
    CHECK(a.value() == doctest::Approx(0.0));
}

TEST_CASE("SlideAnimator: overshooting dt pins exactly to target and marks done") {
    SlideAnimator a;
    a.start(0.0, 50.0, 0.05, linear()); // 50ms run
    // First tick's dt (0.1s) exceeds the run AND is exactly the clamp; elapsed
    // becomes 0.1 >= 0.05 so it completes this frame, pinned to `to`.
    CHECK(a.tick(0.1) == doctest::Approx(50.0));
    CHECK_FALSE(a.active());
}

TEST_CASE("SlideAnimator: dt is clamped (kMaxTickDt) so a huge frame gap doesn't teleport") {
    SlideAnimator a;
    a.start(0.0, 100.0, 1.0, linear());
    // A 5-second stall: dt is clamped to kMaxTickDt (0.1s), so the run advances
    // only 10% this frame — the motion stays visible instead of snapping to 100.
    double v = a.tick(5.0);
    CHECK(v == doctest::Approx(10.0)); // 0.1s / 1.0s = 10%
    CHECK(a.active());
}

TEST_CASE("SlideAnimator: a non-linear tween is honoured and sampled at t=0 and t=1") {
    // An ease-out-ish tween: f(t) = t*(2-t). f(0)=0, f(1)=1, f(0.5)=0.75.
    auto ease_out = [](float t) { return t * (2.0F - t); };

    // Sample at t=0.5: 100 * 0.75 = 75 (ahead of a linear 50 — the ease-out
    // front-loads progress). Use a short 0.5s run so a single dt=0.05 tick (well
    // under the clamp) lands at exactly progress 0.5.
    SlideAnimator half;
    half.start(/*from=*/0.0, /*to=*/100.0, /*duration=*/0.1, ease_out);
    CHECK(half.tick(0.05) == doctest::Approx(75.0)); // progress 0.5 -> eased 0.75

    // Endpoints: t=0 maps to `from`, t=1 maps EXACTLY to `to` (the done-pin,
    // independent of the tween). Re-run to sample the start endpoint cleanly.
    SlideAnimator b;
    b.start(0.0, 100.0, 0.1, ease_out);
    CHECK(b.value() == doctest::Approx(0.0)); // t=0 -> from, regardless of ease
    CHECK(b.tick(0.1) == doctest::Approx(100.0)); // t=1 -> to (exact, pinned)
    CHECK_FALSE(b.active());
}

TEST_CASE("SlideAnimator: a null ease falls back to linear") {
    SlideAnimator a;
    a.start(0.0, 100.0, /*duration=*/0.1, /*ease=*/nullptr);
    CHECK(a.tick(0.025) == doctest::Approx(25.0)); // progress 0.25, pure linear
}

TEST_CASE("SlideAnimator: non-positive duration snaps immediately to target") {
    SlideAnimator a;
    a.start(-100.0, 0.0, /*duration=*/0.0, linear());
    CHECK_FALSE(a.active()); // nothing to animate
    CHECK(a.value() == doctest::Approx(0.0)); // landed on the target at once
    CHECK(a.tick(0.016) == doctest::Approx(0.0)); // inert thereafter
}

TEST_CASE("SlideAnimator: set_immediate snaps the value and cancels any run") {
    SlideAnimator a;
    a.start(0.0, 100.0, 1.0, linear());
    a.tick(0.3);
    CHECK(a.active());
    // The drag-scrub: snap the value to the finger position with no run.
    a.set_immediate(42.0);
    CHECK_FALSE(a.active());
    CHECK(a.value() == doctest::Approx(42.0));
    // tick is inert after set_immediate (the finger drives, not the clock).
    CHECK(a.tick(0.1) == doctest::Approx(42.0));
    CHECK_FALSE(a.active());
}

TEST_CASE("SlideAnimator: interrupting a run re-anchors continuously from the new from") {
    SlideAnimator a;
    // Short run so single sub-clamp ticks land at clean fractions.
    a.start(-100.0, 0.0, 0.1, linear());
    a.tick(0.04); // 40% of the way: -100 + 40 = -60
    CHECK(a.value() == doctest::Approx(-60.0));
    // Reverse mid-run (a drag-release back toward closed) from the CURRENT value:
    // motion continues from -60, not from a stale anchor.
    a.start(a.value(), -100.0, 0.1, linear());
    CHECK(a.value() == doctest::Approx(-60.0)); // continuous: no jump on restart
    CHECK(a.target() == doctest::Approx(-100.0));
    a.tick(0.05); // halfway from -60 to -100 -> -80
    CHECK(a.value() == doctest::Approx(-80.0));
}