fix(imgui): keep the plot inside the child's clip rect and reject unusable input

The widget drew a region derived from the child's content rect alone, so a child
the hosting window cut off cleared and painted over the host's pixels; it also
scaled ImGui's (-FLT_MAX, -FLT_MAX) no-pointer sentinel and differenced it against
the last real position, turning a drag out of the window into a ~1e35 pan, and it
forgot the host's button levels across a re-bind, handing the next plot a press
that never happened.

The region is now the content rect intersected with the child's clip rect (a
no-op when nothing clips it), an invalid position is no position update, and the
button levels survive a re-bind. The controller sanitises non-finite coordinates
and deltas for any host, and the widget's input translation now has a committed
test on the headless EGL context.

CMakeLists.txt

@@ -34,21 +34,13 @@ target_compile_features(plot_core PUBLIC cxx_std_20)
 target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})
 plot_apply_warnings(plot_core)
 
-if(PLOT_BUILD_TESTS)
-  # Test-only: counts the pyramid's covering-decomposition entries and its bucket
-  # merges so a test can pin the query's O(log n) cost and the append path's
-  # amortised O(1) cost, neither of which any answer reveals. Off in every
-  # non-test build (see include/plot/data/detail/{cover,merge}_count.hpp).
-  target_compile_definitions(plot_core PRIVATE PLOT_PYRAMID_COUNT_COVERS
-                                             PLOT_PYRAMID_COUNT_MERGES)
-
-  enable_testing()
-  add_subdirectory(tests)
-endif()
-
 # Adapters own the surface and translate their host's events; examples link the
-# adapter they demo. Both are opt-in, so the core library and its tests still
+# adapter they demo. Both are opt-in, so the core library and its tests still build
-# build on a bare toolchain.
+# on a bare toolchain.
+#
+# They are added before the test suite on purpose: a test that drives an adapter
+# needs that adapter's targets to exist, and with both switches on the suite can
+# then exercise them (see tests/CMakeLists.txt).
 if(PLOT_BUILD_GLFW)
   add_subdirectory(adapters/glfw)
 endif()
@@ -60,3 +52,15 @@ endif()
 if(PLOT_BUILD_EXAMPLES)
   add_subdirectory(examples)
 endif()
+
+if(PLOT_BUILD_TESTS)
+  # Test-only: counts the pyramid's covering-decomposition entries and its bucket
+  # merges so a test can pin the query's O(log n) cost and the append path's
+  # amortised O(1) cost, neither of which any answer reveals. Off in every
+  # non-test build (see include/plot/data/detail/{cover,merge}_count.hpp).
+  target_compile_definitions(plot_core PRIVATE PLOT_PYRAMID_COUNT_COVERS
+                                             PLOT_PYRAMID_COUNT_MERGES)
+
+  enable_testing()
+  add_subdirectory(tests)
+endif()

adapters/imgui/imgui_plot.cpp

@@ -67,27 +67,39 @@ void PlotWidget::draw(const char* id, Plot& plot, Vec2 size, Renderer& renderer)
     return;
   }
 
-  // A borderless child has no padding and no border, so the content region is
+  // A borderless child has no padding and no border, so the content region is the
-  // the child's rectangle itself: the pixels the plot draws are exactly the ones
+  // child's rectangle itself: the pixels the plot draws are exactly the ones the
-  // the host laid out for it.
+  // host laid out for it -- if the host leaves them visible. ImGui hands a child
+  // the clip rect of an ancestor that cuts it off (and the child's own rect when
+  // nothing does, which makes the intersection below a no-op); the widget's GL
+  // draws do not go through ImGui's clipping, so a child the hosting window cuts
+  // off would otherwise clear and paint over whatever the host drew outside it.
   const ImVec2 origin = ImGui::GetCursorScreenPos();
   const ImVec2 avail = ImGui::GetContentRegionAvail();
-  if (avail.x <= 0.0f || avail.y <= 0.0f) {
+  const ImVec2 clip_min = ImGui::GetWindowDrawList()->GetClipRectMin();
-    // A child with no room (a degenerate layout): there is no region to draw
+  const ImVec2 clip_max = ImGui::GetWindowDrawList()->GetClipRectMax();
-    // into and no rect to aim a gesture at, and InvisibleButton wants a size.
+  const f32 x0 = origin.x > clip_min.x ? origin.x : clip_min.x;
+  const f32 y0 = origin.y > clip_min.y ? origin.y : clip_min.y;
+  const f32 x1 = origin.x + avail.x < clip_max.x ? origin.x + avail.x : clip_max.x;
+  const f32 y1 = origin.y + avail.y < clip_max.y ? origin.y + avail.y : clip_max.y;
+  if (x1 <= x0 || y1 <= y0) {
+    // Nothing of the child is visible: no region to draw into, no rect to aim a
+    // gesture at, and InvisibleButton wants a non-zero size.
     ImGui::EndChild();
     ImGui::PopStyleColor();
     return;
   }
+  const ImVec2 visible{x1 - x0, y1 - y0};
 
-  // An invisible item covering the region is what makes it *the* hovered thing:
+  // An invisible item covering the *visible* region is what makes it *the* hovered
-  // `IsItemHovered()` is false while a drag elsewhere in the host is active, so
+  // thing: `IsItemHovered()` is false while a drag elsewhere in the host is active,
-  // the plot does not pan when the user drags a slider next to it, and
+  // so the plot does not pan when the user drags a slider next to it, and
   // `SetItemKeyOwner()` -- ImGui's documented way for a widget to take the wheel
   // away from window scrolling -- needs an item to own it. Both buttons are
   // enabled: the controller's left and right drags are the plot's own, and ImGui
   // only needs to know that the pointer is interacting with this region.
-  ImGui::InvisibleButton("surface", ImVec2(avail.x, avail.y),
+  ImGui::SetCursorScreenPos(ImVec2(x0, y0));
+  ImGui::InvisibleButton("surface", visible,
                          ImGuiButtonFlags_MouseButtonLeft | ImGuiButtonFlags_MouseButtonRight |
                              ImGuiButtonFlags_MouseButtonMiddle);
   const bool hovered = ImGui::IsItemHovered();
@@ -98,9 +110,9 @@ void PlotWidget::draw(const char* id, Plot& plot, Vec2 size, Renderer& renderer)
   const f32 fb_h_f = io.DisplaySize.y * scale.y;
   // Points -> framebuffer pixels: the assumption documented on the class. State
   // it in the arithmetic so a reader sees where the mapping enters.
-  const Rect region{origin.x * scale.x, origin.y * scale.y, avail.x * scale.x, avail.y * scale.y};
+  const Rect region{x0 * scale.x, y0 * scale.y, visible.x * scale.x, visible.y * scale.y};
 
-  if (region.w > 0.0f && region.h > 0.0f && fb_w_f >= 1.0f && fb_h_f >= 1.0f) {
+  if (fb_w_f >= 1.0f && fb_h_f >= 1.0f) {
     // One viewport for the gesture geometry and the frame: the controller
     // forwards it to the plot (and re-lays it out), and `begin_frame` gets the
     // same rect and the whole framebuffer it is a sub-rectangle of.
@@ -125,29 +137,46 @@ void PlotWidget::forward_input(bool hovered) {
   ImGuiIO& io = ImGui::GetIO();
   const ImVec2 scale = io.DisplayFramebufferScale;
 
-  const f32 px = io.MousePos.x * scale.x;
+  // ImGui reports "the pointer is not on this window" as a sentinel position:
-  const f32 py = io.MousePos.y * scale.y;
+  // imgui_impl_glfw's cursor-enter callback leaves (-FLT_MAX, -FLT_MAX) when the
+  // pointer leaves, and ClearInputMouse() does the same on a lost focus. Scaling
+  // that and differencing it against the last real position would hand the
+  // controller a ~1e35 "movement" for that frame -- and with a press captured over
+  // the plot, that movement is a pan, so the range would leave the representable
+  // world (and only `f` would bring it back, since a gesture that sets a range also
+  // clears auto scale). An invalid position is therefore no position *update*: the
+  // last real one stands, the movement is zero, and the button levels below still
+  // flow, so the release that ends a drag is never lost.
+  const bool pos_valid = ImGui::IsMousePosValid(&io.MousePos);
+  const f32 px = pos_valid ? io.MousePos.x * scale.x : prev_x_;
+  const f32 py = pos_valid ? io.MousePos.y * scale.y : prev_y_;
+
   const bool down[3] = {io.MouseDown[0], io.MouseDown[1], io.MouseDown[2]};
   const bool any_down = down[0] || down[1] || down[2];
   const u32 mods = mods_from(io);
 
-  if (!hovered && !capturing_) {
+  if ((!pos_valid && !have_prev_pos_) || (!hovered && !capturing_)) {
-    // Nowhere near the plot: remember the levels anyway, so re-entering the
+    // Either no position has ever been reported (nothing to aim at, no movement
-    // region does not synthesise an edge out of the state it left with.
+    // to forward), or the pointer is nowhere near the plot. Remember the levels
+    // either way, so re-entering the region does not synthesise an edge out of the
+    // state it left with -- and so a re-bound plot, which keeps its state too,
+    // cannot fabricate one either.
     prev_down_[0] = down[0];
     prev_down_[1] = down[1];
     prev_down_[2] = down[2];
+    if (pos_valid) {
       prev_x_ = px;
       prev_y_ = py;
       have_prev_pos_ = true;
+    }
     return;
   }
 
   // The movement since the previous frame is what a pan consumes; it rides the
   // first event of this frame and nothing else, so one frame's motion can never
-  // be counted twice.
+  // be counted twice. An invalid position contributes none of it.
-  const f32 dx = have_prev_pos_ ? px - prev_x_ : 0.0f;
+  const f32 dx = (pos_valid && have_prev_pos_) ? px - prev_x_ : 0.0f;
-  const f32 dy = have_prev_pos_ ? py - prev_y_ : 0.0f;
+  const f32 dy = (pos_valid && have_prev_pos_) ? py - prev_y_ : 0.0f;
   bool delta_pending = true;
 
   const auto deliver = [&](input::Button button, bool pressed, bool released) {
@@ -216,18 +245,23 @@ void PlotWidget::forward_input(bool hovered) {
   prev_down_[0] = down[0];
   prev_down_[1] = down[1];
   prev_down_[2] = down[2];
+  if (pos_valid) {
     prev_x_ = px;
     prev_y_ = py;
     have_prev_pos_ = true;
+  }
   // Capture: a press delivered while the pointer was over the child keeps the
   // release flowing even once the pointer has left it.
   capturing_ = any_down && (capturing_ || hovered);
 }
 
 void PlotWidget::reset_pointer() noexcept {
-  prev_down_[0] = false;
+  // The remembered button levels deliberately survive a re-bind, because they are
-  prev_down_[1] = false;
+  // the host's input state and not the plot's: clearing them would make the next
-  prev_down_[2] = false;
+  // frame synthesise a press out of a button that is already down, and a host
+  // drawing two plots through one widget re-binds twice a frame. Only the
+  // position memory and the capture are dropped, so the new plot's first event
+  // carries no movement carried over from the old one.
   prev_x_ = 0.0f;
   prev_y_ = 0.0f;
   have_prev_pos_ = false;

adapters/imgui/imgui_plot.hpp

@@ -66,6 +66,12 @@ namespace plot::imgui {
 /// The child's own background is forced transparent by the widget, whatever the
 /// host's `ImGuiCol_ChildBg`, so the widget itself never paints over the plot.
 ///
+/// The converse also holds: what ImGui clips away, the widget does not paint. GL
+/// draws do not go through ImGui's clipping, so the region is intersected with the
+/// clip rect ImGui gives the child (its own rectangle when nothing cuts it off,
+/// the visible part of it when an ancestor does) -- a child the hosting window cuts
+/// off must not clear and paint over whatever the host drew beside it.
+///
 /// ## Input
 ///
 /// Pointer, wheel and key state are translated into `plot::input` events and
@@ -74,13 +80,21 @@ namespace plot::imgui {
 /// controller wants edges (`PointerEvent::pressed`/`released`), so the widget
 /// remembers the previous frame's buttons and synthesises the transitions.
 ///
-/// Events are forwarded while an invisible item covering the child's region is
+/// Events are forwarded while an invisible item covering the child's visible
-/// hovered -- `IsItemHovered()` rather than `IsWindowHovered()`, because the
+/// region is hovered -- `IsItemHovered()` rather than `IsWindowHovered()`, because
-/// item is the child's whole rectangle (the child has no padding) and its hover
+/// the item is the child's whole rectangle (the child has no padding) and its hover
-/// is false while the host is dragging some other widget, so dragging a slider
+/// is false while the host is dragging some other widget, so dragging a slider next
-/// next to the plot cannot pan it. A press that started over the child keeps
+/// to the plot cannot pan it. A press that started over the child keeps being
-/// being delivered until every button is up again, so a drag that leaves the
+/// delivered until every button is up again, so a drag that leaves the region still
-/// region still ends instead of leaving the plot mid-pan.
+/// ends instead of leaving the plot mid-pan.
+///
+/// An *invalid* pointer position -- ImGui's `(-FLT_MAX, -FLT_MAX)` sentinel for "the
+/// pointer is not on this window", which the platform backends leave behind when the
+/// pointer leaves or the window loses focus -- is no position update: the last real
+/// position stands, the movement for that frame is zero (a sentinel scaled by the
+/// framebuffer scale and differenced against a real position would be a ~1e35 pan),
+/// and the button levels still flow, so a drag that leaves the window still ends on
+/// its release.
 ///
 /// Wheel input goes to the plot while it is hovered, and the plot's zoom wins
 /// over ImGui's window scrolling: `SetItemKeyOwner(ImGuiKey_MouseWheelY)` on that
@@ -109,15 +123,19 @@ class PlotWidget {
   /// window; the plot is drawn (GL calls issued) during the call, inside the
   /// child, so the host's later ImGui commands never have to restore anything.
   ///
-  /// The plot's viewport is the child's content rectangle, so it fills the child
+  /// The plot's viewport is the child's visible rectangle, so it fills the child
   /// exactly; the widget sets it every frame through the controller, and the plot
-  /// is re-laid out to match. A degenerate region (a collapsed or clipped child)
+  /// is re-laid out to match. A degenerate or entirely
-  /// draws nothing and forwards nothing.
+  /// clipped region draws nothing and forwards nothing, and a partially clipped one
+  /// draws and accepts gestures only where the host leaves the child visible.
   void draw(const char* id, Plot& plot, Vec2 size, Renderer& renderer);
 
   /// The controller the widget feeds, or null before the first `draw()`. It is
   /// bound to whichever plot the last `draw()` was given; a `draw()` for a
-  /// different plot re-binds it and drops any drag in progress.
+  /// different plot re-binds it and drops any drag in progress. The remembered
+  /// button levels survive the re-bind -- they are the host's state, not the
+  /// plot's -- so a host drawing two plots through one widget does not hand the
+  /// second one a fabricated press.
   input::Controller* controller() noexcept { return controller_ ? &*controller_ : nullptr; }
   const input::Controller* controller() const noexcept {
     return controller_ ? &*controller_ : nullptr;
@@ -127,8 +145,9 @@ class PlotWidget {
   /// Synthesises this frame's `plot::input` events from ImGui's levels.
   void forward_input(bool hovered);
 
-  /// Forgets the previous frame's pointer state, so a re-bound plot cannot see an
+  /// Drops the remembered pointer position and any capture, keeping the remembered
-  /// edge that belongs to the old one.
+  /// button levels: they are the host's state, and synthesising an edge out of the
+  /// levels a re-bind started with would fabricate a press.
   void reset_pointer() noexcept;
 
   Plot* plot_ = nullptr;

include/plot/input/controller.hpp

@@ -58,6 +58,13 @@ class Controller {
   /// refreshes the layout, so gesture geometry and the host agree.
   void set_viewport(Rect px);
 
+  /// Events are sanitised before they are acted on: a non-finite coordinate or
+  /// delta -- a host's "no pointer" sentinel (ImGui reports `-FLT_MAX`), a NaN
+  /// from a degenerate mapping, an infinity from a lost device -- becomes the last
+  /// known position and no movement. Garbage must not reach the ranges: a gesture
+  /// that sets a range also clears auto scale, so a range driven to 1e35 is one the
+  /// user can neither pan nor zoom back. The buttons in the same event are still
+  /// acted on, so a drag whose release arrives with a bad position still ends.
   void on_pointer(const PointerEvent& e);
   void on_wheel(const WheelEvent& e);
   void on_key(const KeyEvent& e);

src/input/controller.cpp

@@ -39,6 +39,44 @@ f64 from_space(f64 v, bool log) noexcept {
 /// key) zoom in, so the span shrinks by `exp(-notches * kZoomPerNotch)`.
 f64 zoom_factor(f64 notches) noexcept { return std::exp(-notches * kZoomPerNotch); }
 
+/// True when a coordinate is one arithmetic can work with. Every event here comes
+/// from a host, and a host can hand over anything: a toolkit's "no pointer" 
+/// sentinel, a NaN from a degenerate mapping of its own, an infinity from a lost
+/// device. A non-finite value rides the pan and zoom arithmetic into a range that
+/// is no longer a range, and since a gesture that sets a range also clears auto
+/// scale, `f` would be the only way back -- so it must not get in.
+bool finite(f32 v) noexcept { return std::isfinite(v); }
+
+/// `e`, with an unusable position replaced by the last known one and an unusable
+/// delta by no movement. Sanitising rather than rejecting keeps the buttons in the
+/// same event alive: a drag whose release arrived with a garbage position must
+/// still end.
+PointerEvent sanitized(const PointerEvent& e, bool have_pointer, f32 last_x,
+                       f32 last_y) noexcept {
+  PointerEvent in = e;
+  if (!finite(in.x) || !finite(in.y)) {
+    in.x = have_pointer ? last_x : 0.0f;
+    in.y = have_pointer ? last_y : 0.0f;
+    in.dx = 0.0f;
+    in.dy = 0.0f;
+    return in;
+  }
+  if (!finite(in.dx)) in.dx = 0.0f;
+  if (!finite(in.dy)) in.dy = 0.0f;
+  return in;
+}
+
+WheelEvent sanitized(const WheelEvent& e, bool have_pointer, f32 last_x, f32 last_y) noexcept {
+  WheelEvent in = e;
+  if (!finite(in.x) || !finite(in.y)) {
+    in.x = have_pointer ? last_x : 0.0f;
+    in.y = have_pointer ? last_y : 0.0f;
+  }
+  if (!finite(in.dx)) in.dx = 0.0f;
+  if (!finite(in.dy)) in.dy = 0.0f;
+  return in;
+}
+
 }  // namespace
 
 void Controller::set_viewport(Rect px) {
@@ -49,7 +87,8 @@ void Controller::set_viewport(Rect px) {
   plot_->layout();
 }
 
-void Controller::on_pointer(const PointerEvent& e) {
+void Controller::on_pointer(const PointerEvent& raw) {
+  const PointerEvent e = sanitized(raw, has_pointer_, pointer_x_, pointer_y_);
   has_pointer_ = true;
   pointer_x_ = e.x;
   pointer_y_ = e.y;
@@ -66,7 +105,8 @@ void Controller::on_pointer(const PointerEvent& e) {
   if (e.down && mode_ != DragMode::None) update_drag(e);
 }
 
-void Controller::on_wheel(const WheelEvent& e) {
+void Controller::on_wheel(const WheelEvent& raw) {
+  const WheelEvent e = sanitized(raw, has_pointer_, pointer_x_, pointer_y_);
   plot_->layout();
   const f64 factor = zoom_factor(static_cast<f64>(e.dy));
   if (!(factor > 0.0)) return;

tests/CMakeLists.txt

@@ -40,6 +40,17 @@ add_executable(plot_tests
   egl_headless.cpp
 )
 target_link_libraries(plot_tests PRIVATE plot::core ${plot_egl_library} Threads::Threads)
+
+# The Dear ImGui widget is the one adapter with a test here: its input translation
+# (edge synthesis, the hover gate, drag capture, wheel containment) is logic this
+# project owns, not a toolkit's. The suite is otherwise ImGui-free, so the source
+# and the link are added only when the adapter exists -- which the top-level
+# CMakeLists.txt guarantees by adding the adapters before this directory.
+if(TARGET plot::imgui)
+  target_sources(plot_tests PRIVATE test_imgui_widget.cpp)
+  target_link_libraries(plot_tests PRIVATE plot::imgui imgui)
+endif()
+
 # Same switches the library is built with: the cost tests read the counters
 # behind them (see include/plot/data/detail/{cover,merge}_count.hpp).
 target_compile_definitions(plot_tests PRIVATE PLOT_PYRAMID_COUNT_COVERS

tests/test_controller.cpp

@@ -5,6 +5,7 @@
 #include <plot/data/array_series.hpp>
 
 #include <cmath>
+#include <limits>
 #include <utility>
 #include <vector>
 
@@ -248,6 +249,89 @@ PLOT_TEST(key_minus_zooms_out) {
   PLOT_CHECK(f.plot.x_range(f.ax).span() > 100.0);
 }
 
+/// A host can hand the controller a coordinate that is not a number: ImGui reports
+/// (-FLT_MAX, -FLT_MAX) for "no mouse position", and any host can produce a NaN out
+/// of a degenerate mapping of its own. The pan arithmetic runs through the axis
+/// transforms, so such a value would leave a range that is not a range -- and
+/// because a gesture that sets a range also clears auto scale, panning and zooming
+/// could not bring it back.
+PLOT_TEST(non_finite_pointer_does_not_move_a_range) {
+  plot::Plot p;
+  p.set_viewport({0, 0, 800, 400});
+  const u32 ax = p.add_x_axis({"t", {0.0, 100.0}, true, plot::AxisPlace::Bottom, false});
+  const u32 ay = p.add_y_axis({"v", {-1.0, 1.0}, true, plot::AxisPlace::Left, false});
+  plot::input::Controller c(p);
+  c.set_viewport({0, 0, 800, 400});
+  const plot::Range xr = p.x_range(ax);
+  const plot::Range yr = p.y_range(ay);
+
+  c.on_pointer({100, 100, 0, 0, plot::input::Button::Left, 0, true, true, false});
+  PLOT_CHECK(c.drag_mode() == plot::input::DragMode::Pan);
+
+  const float inf = std::numeric_limits<float>::infinity();
+  const float nan = std::numeric_limits<float>::quiet_NaN();
+  const float sentinel = -std::numeric_limits<float>::max();  // ImGui's "no pointer"
+
+  // The pointer-left-the-window sentinel arrives at the unrepresentable end of the
+  // axis, so scaling and differencing it is a 1e35 pan. It must read as no
+  // movement at all.
+  c.on_pointer({sentinel, sentinel, inf, -inf, plot::input::Button::Left, 0, true, false, false});
+  PLOT_CHECK(std::isfinite(p.x_range(ax).min) && std::isfinite(p.x_range(ax).max));
+  PLOT_CHECK(std::isfinite(p.y_range(ay).min) && std::isfinite(p.y_range(ay).max));
+  PLOT_CHECK_NEAR(p.x_range(ax).min, xr.min, 1e-9);
+  PLOT_CHECK_NEAR(p.x_range(ax).max, xr.max, 1e-9);
+  PLOT_CHECK_NEAR(p.y_range(ay).min, yr.min, 1e-9);
+  PLOT_CHECK(c.drag_mode() == plot::input::DragMode::Pan);  // still the same drag
+
+  // A usable position with an unusable delta is no movement either, and a later
+  // usable position is not differenced against the sentinel.
+  c.on_pointer({180, 130, inf, nan, plot::input::Button::Left, 0, true, false, false});
+  PLOT_CHECK_NEAR(p.x_range(ax).min, xr.min, 1e-9);
+  PLOT_CHECK_NEAR(p.x_range(ax).max, xr.max, 1e-9);
+  PLOT_CHECK_NEAR(p.y_range(ay).min, yr.min, 1e-9);
+  c.on_pointer({220, 160, 40, 30, plot::input::Button::Left, 0, true, false, false});
+  PLOT_CHECK(std::isfinite(p.x_range(ax).min));  // a real move still pans
+  PLOT_CHECK(p.x_range(ax).min < xr.min);
+
+  // The buttons in a sanitised event still act: the release ends the drag.
+  c.on_pointer({220, 160, 0, 0, plot::input::Button::Left, 0, false, false, true});
+  PLOT_CHECK(c.drag_mode() == plot::input::DragMode::None);
+}
+
+PLOT_TEST(non_finite_wheel_does_not_break_a_range) {
+  plot::Plot p;
+  p.set_viewport({0, 0, 800, 400});
+  const u32 ax = p.add_x_axis({"t", {0.0, 100.0}, true, plot::AxisPlace::Bottom, false});
+  const u32 ay = p.add_y_axis({"v", {-1.0, 1.0}, true, plot::AxisPlace::Left, false});
+  plot::input::Controller c(p);
+  c.set_viewport({0, 0, 800, 400});
+
+  const float inf = std::numeric_limits<float>::infinity();
+  const float nan = std::numeric_limits<float>::quiet_NaN();
+
+  // A zoom is anchored at the data coordinate under the pointer, so a position that
+  // is not a number must not become the anchor: the axis would be re-centred on
+  // NaN, which no gesture could undo. The fallback is a real coordinate and the
+  // zoom still happens. (A *finite* sentinel -- ImGui's `-FLT_MAX` for "no pointer"
+  // -- is not the controller's to interpret: finite coordinates are legal input.
+  // The adapter translates that one; see tests/test_imgui_widget.cpp.)
+  const double span_before = p.x_range(ax).span();
+  c.on_wheel({nan, nan, 0.0f, 1.0f, 0});
+  PLOT_CHECK(std::isfinite(p.x_range(ax).min) && std::isfinite(p.x_range(ax).max));
+  PLOT_CHECK(std::isfinite(p.y_range(ay).min) && std::isfinite(p.y_range(ay).max));
+  PLOT_CHECK(std::fabs(p.x_range(ax).min) < 1e6 && std::fabs(p.x_range(ax).max) < 1e6);
+  PLOT_CHECK(p.x_range(ax).span() < span_before);
+
+  // A non-finite amount is no zoom at all.
+  const plot::Range xr = p.x_range(ax);
+  const plot::Range yr = p.y_range(ay);
+  c.on_wheel({400.0f, 200.0f, 0.0f, nan, 0});
+  c.on_wheel({400.0f, 200.0f, 0.0f, inf, 0});
+  PLOT_CHECK_NEAR(p.x_range(ax).min, xr.min, 1e-12);
+  PLOT_CHECK_NEAR(p.x_range(ax).max, xr.max, 1e-12);
+  PLOT_CHECK_NEAR(p.y_range(ay).min, yr.min, 1e-12);
+}
+
 PLOT_TEST(key_f_auto_fits_traces) {
   plot::Plot p;
   p.set_viewport({0, 0, 800, 400});

tests/test_imgui_widget.cpp

@@ -0,0 +1,440 @@
+// The Dear ImGui adapter's own logic, pinned against real ImGui frames on the
+// tests' surfaceless EGL context -- no window, no display server.
+//
+// What is tested here is what this project owns, not the toolkit's: ImGui reports
+// input as *levels* where `plot::input` wants edges, so the widget synthesises
+// them, gates delivery on hover, captures a drag across a region exit, and takes
+// the wheel away from window scrolling; and because it draws through GL rather
+// than ImGui's draw list, it has to keep its painting inside the child's *visible*
+// rectangle itself. The suite is otherwise ImGui-free, so this file and its link
+// are added only when the adapter is built (tests/CMakeLists.txt).
+
+#include "harness.hpp"
+#include "egl_headless.hpp"
+
+#include <imgui/imgui_plot.hpp>
+
+#include <imgui.h>
+
+#include <plot/data/stream_series.hpp>
+#include <plot/gl/api.hpp>
+#include <plot/render/renderer.hpp>
+
+#include <cmath>
+#include <cstdint>
+#include <limits>
+#include <utility>
+#include <vector>
+
+namespace {
+
+/// The off-screen target and the ImGui display are the same size: the widget's
+/// region is a sub-rectangle of the framebuffer, which is the point of the test.
+constexpr u32 kFb = 512;
+constexpr float kDisplay = 512.0f;
+constexpr float kChild = 400.0f;
+
+/// A plot the widget drives, and the host frame around it.
+struct Fixture {
+  plottest::HeadlessContext ctx;
+  plot::Renderer renderer;
+  plot::data::StreamSeries series{4096, 0.0, 0.01};
+  plot::Plot plot;
+  plot::imgui::PlotWidget widget;
+  u32 ax = 0;
+  u32 ay = 0;
+  /// A second plot, so a host that draws two through one widget can be tested.
+  plot::Plot other;
+  u32 ax_other = 0;
+  u32 ay_other = 0;
+
+  float win_w = kDisplay;
+  float win_h = kDisplay;
+  float child_w = kChild;
+  float child_h = kChild;
+  /// Content past the window, so the hosting window has a scroll range: the wheel
+  /// containment test needs one to be able to observe a scroll at all.
+  bool tall_parent = false;
+  bool prev_down = false;
+
+  ImVec2 child_origin{0.0f, 0.0f};  ///< the child's top-left, read in-frame (points)
+  float scroll_y = 0.0f;            ///< the hosting window's scroll, read in-frame
+
+  /// The pixels the widget painted: the bounding box of everything that is not the
+  /// host's clear colour, in top-left framebuffer coordinates.
+  u32 paint_min_x = kFb;
+  u32 paint_min_y = kFb;
+  u32 paint_max_x = 0;
+  u32 paint_max_y = 0;
+
+  /// The host's own frame. Green on purpose: no colour the plot draws, nor any
+  /// blend of them, is (0, 192, 0), so "the host's pixel survived" is decidable.
+  plot::Color host_color = plot::Color::hex(0x00C000);
+  std::uint8_t host_rgb_[3] = {0, 0, 0};
+
+  Fixture() : ctx(kFb, kFb) {}
+  ~Fixture() { ImGui::DestroyContext(); }
+
+  Fixture(const Fixture&) = delete;
+  Fixture& operator=(const Fixture&) = delete;
+
+  bool init() {
+    plot::Status st;
+    if (!ctx.create(&st)) {
+      std::printf("[FAIL] headless context: %s\n", st.message);
+      return false;
+    }
+    if (!renderer.init(&st)) {
+      std::printf("[FAIL] renderer init: %s\n", st.message);
+      return false;
+    }
+
+    ImGui::CreateContext();
+    ImGuiIO& io = ImGui::GetIO();
+    io.IniFilename = nullptr;
+    io.DisplaySize = ImVec2(kDisplay, kDisplay);
+    io.DisplayFramebufferScale = ImVec2(1.0f, 1.0f);
+    io.Fonts->AddFontDefault();
+    std::uint8_t* pixels = nullptr;
+    int tw = 0;
+    int th = 0;
+    // The widget needs a built atlas exactly as a platform backend would leave it;
+    // no renderer backend is involved, so the texture id is a placeholder.
+    io.Fonts->GetTexDataAsRGBA32(&pixels, &tw, &th);
+    io.Fonts->SetTexID(static_cast<ImTextureID>(1));
+
+    for (int i = 0; i < 4096; ++i) series.push(static_cast<f32>(std::sin(0.05 * i)));
+    series.acquire();
+    ax = plot.add_x_axis({"time", {0.0, 20.0}, false, plot::AxisPlace::Bottom, false});
+    ay = plot.add_y_axis({"signal", {-1.2, 1.2}, false, plot::AxisPlace::Left, false});
+    plot.add_trace({&series, plot::Color::hex(0x40C0FF), 1.5f, ax, ay});
+    plot.set_x_range(ax, {0.0, 20.0});  // explicit: clears auto scale, so a pan shows
+    plot.set_y_range(ay, {-1.2, 1.2});
+
+    other.set_viewport(plot.viewport());
+    ax_other = other.add_x_axis({"time", {0.0, 50.0}, false, plot::AxisPlace::Bottom, false});
+    ay_other = other.add_y_axis({"signal", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+    other.add_trace({&series, plot::Color::hex(0xFFB020), 1.5f, ax_other, ay_other});
+    other.set_x_range(ax_other, {0.0, 50.0});
+    other.set_y_range(ay_other, {-1.0, 1.0});
+
+    // The host colour as the pipeline actually produced it: the pixel tests compare
+    // bytes, not a rounding of the clear value.
+    clear_host();
+    std::uint8_t probe[4] = {0, 0, 0, 0};
+    pixel(0, 0, probe);
+    host_rgb_[0] = probe[0];
+    host_rgb_[1] = probe[1];
+    host_rgb_[2] = probe[2];
+    return true;
+  }
+
+  void clear_host() {
+    plot::gl::glViewport(0, 0, static_cast<plot::gl::GLsizei>(ctx.width),
+                         static_cast<plot::gl::GLsizei>(ctx.height));
+    plot::gl::glClearColor(host_color.r, host_color.g, host_color.b, host_color.a);
+    plot::gl::glClear(plot::gl::GL_COLOR_BUFFER_BIT);
+  }
+
+  /// One widget frame with the host's input queued exactly as a platform backend
+  /// would queue it. `pos` is in ImGui points; `-FLT_MAX` is ImGui's "the pointer is
+  /// not on this window" sentinel.
+  void frame(const ImVec2& pos, bool down, float wheel = 0.0f, plot::Plot* target = nullptr) {
+    plot::Plot& shown = target != nullptr ? *target : plot;
+    ImGuiIO& io = ImGui::GetIO();
+    io.AddMousePosEvent(pos.x, pos.y);
+    if (down != prev_down) {
+      io.AddMouseButtonEvent(0, down);
+      prev_down = down;
+    }
+    if (wheel != 0.0f) io.AddMouseWheelEvent(0.0f, wheel);
+
+    // A bare window: no background to paint over the plot, and no title bar or
+    // scrollbar to add pixels the scan below would mistake for the widget's.
+    const ImGuiWindowFlags host_flags = ImGuiWindowFlags_NoBackground |
+                                       ImGuiWindowFlags_NoTitleBar |
+                                       ImGuiWindowFlags_NoScrollbar;
+    ImGui::NewFrame();
+    ImGui::SetNextWindowPos(ImVec2(0.0f, 0.0f), ImGuiCond_Always);
+    ImGui::SetNextWindowSize(ImVec2(win_w, win_h), ImGuiCond_Always);
+    ImGui::Begin("host", nullptr, host_flags);
+    child_origin = ImGui::GetCursorScreenPos();  // where the widget's child goes
+    widget.draw("plot", shown, plot::Vec2{child_w, child_h}, renderer);
+    if (tall_parent) ImGui::Dummy(ImVec2(10.0f, 900.0f));  // content past the window
+    scroll_y = ImGui::GetScrollY();
+    ImGui::End();
+    ImGui::Render();
+  }
+
+  /// The framebuffer pixel at a *top-left* coordinate. `read_pixel` speaks GL's
+  /// bottom-left convention, as the render smoke test notes.
+  void pixel(u32 x, u32 y, std::uint8_t out[4]) {
+    if (!ctx.read_pixel(x, ctx.height - 1u - y, out)) PLOT_CHECK(false);
+  }
+
+  bool is_host(const std::uint8_t px[4]) const {
+    return px[0] == host_rgb_[0] && px[1] == host_rgb_[1] && px[2] == host_rgb_[2];
+  }
+
+  /// Records the bounding box of the pixels that are not the host's clear colour.
+  /// Reads the whole framebuffer once, so a draw that escaped the widget's region
+  /// shows up even if it is nowhere near the pixels a point check samples.
+  void scan_painted() {
+    std::uint8_t probe[4] = {0, 0, 0, 0};
+    pixel(0, 0, probe);  // read_pixel binds the target the frame was drawn into
+    std::vector<std::uint8_t> rgba(static_cast<plot::usize>(kFb) * kFb * 4u, 0u);
+    plot::gl::glPixelStorei(plot::gl::GL_PACK_ALIGNMENT, 4);
+    plot::gl::glReadPixels(0, 0, static_cast<plot::gl::GLsizei>(ctx.width),
+                           static_cast<plot::gl::GLsizei>(ctx.height), plot::gl::GL_RGBA,
+                           plot::gl::GL_UNSIGNED_BYTE, rgba.data());
+
+    paint_min_x = kFb;
+    paint_min_y = kFb;
+    paint_max_x = 0;
+    paint_max_y = 0;
+    for (u32 y = 0; y < kFb; ++y) {
+      // glReadPixels hands rows back bottom-up; this scan speaks top-left, the
+      // convention every Rect here uses.
+      const std::uint8_t* row =
+          rgba.data() + static_cast<plot::usize>(kFb - 1u - y) * kFb * 4u;
+      for (u32 x = 0; x < kFb; ++x) {
+        if (is_host(row + static_cast<plot::usize>(x) * 4u)) continue;
+        if (x < paint_min_x) paint_min_x = x;
+        if (x > paint_max_x) paint_max_x = x;
+        if (y < paint_min_y) paint_min_y = y;
+        if (y > paint_max_y) paint_max_y = y;
+      }
+    }
+  }
+
+  plot::Range x_range() const { return plot.x_range(ax); }
+  bool panning() const {
+    const plot::input::Controller* c = widget.controller();
+    return c != nullptr && c->drag_mode() == plot::input::DragMode::Pan;
+  }
+};
+
+constexpr float kMaxFloat = std::numeric_limits<float>::max();
+
+}  // namespace
+
+PLOT_TEST(imgui_widget_pans_only_while_hovered_and_dragging) {
+  Fixture f;
+  if (!f.init()) PLOT_CHECK(false);
+
+  // A hovered pointer with no button moves nothing: the pointer must be *over* the
+  // plot and a drag must be in progress.
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  const plot::Range hovered = f.x_range();
+  f.frame(ImVec2(260.0f, 200.0f), false);
+  PLOT_CHECK_NEAR(f.x_range().min, hovered.min, 1e-9);
+  PLOT_CHECK_NEAR(f.x_range().max, hovered.max, 1e-9);
+
+  // A press begins a pan, but a press alone moves nothing either.
+  f.frame(ImVec2(200.0f, 200.0f), true);
+  PLOT_CHECK(f.panning());
+  PLOT_CHECK_NEAR(f.x_range().min, hovered.min, 1e-9);
+
+  // The drag pans (grab-and-drag: the data follows the cursor) and keeps the span.
+  const plot::Range pressed = f.x_range();
+  f.frame(ImVec2(260.0f, 200.0f), true);
+  PLOT_CHECK(f.x_range().min < pressed.min - 1e-6);
+  PLOT_CHECK_NEAR(f.x_range().span(), pressed.span(), 1e-9);
+
+  // The release ends it, and the next hovered move must not pan again.
+  f.frame(ImVec2(260.0f, 200.0f), false);
+  PLOT_CHECK(!f.panning());
+  const plot::Range released = f.x_range();
+  f.frame(ImVec2(300.0f, 200.0f), false);
+  PLOT_CHECK_NEAR(f.x_range().min, released.min, 1e-9);
+  PLOT_CHECK_NEAR(f.x_range().max, released.max, 1e-9);
+
+  // A press and drag *outside* the child -- the slider-in-the-same-window case --
+  // must not touch the plot at all.
+  f.frame(ImVec2(470.0f, 470.0f), false);
+  f.frame(ImVec2(470.0f, 470.0f), true);
+  f.frame(ImVec2(440.0f, 440.0f), true);
+  PLOT_CHECK_NEAR(f.x_range().min, released.min, 1e-9);
+  f.frame(ImVec2(440.0f, 440.0f), false);
+}
+
+PLOT_TEST(imgui_widget_drag_capture_survives_leaving_the_child) {
+  Fixture f;
+  if (!f.init()) PLOT_CHECK(false);
+
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.frame(ImVec2(200.0f, 200.0f), true);
+  const plot::Range pressed = f.x_range();
+
+  // Leftwards inside the child: the range minimum rises.
+  f.frame(ImVec2(160.0f, 200.0f), true);
+  PLOT_CHECK(f.x_range().min > pressed.min + 1e-6);
+  const plot::Range inside = f.x_range();
+
+  // Rightwards, with the pointer now outside the child: a captured drag keeps
+  // panning, or the gesture would silently stop following the cursor.
+  f.frame(ImVec2(470.0f, 470.0f), true);
+  PLOT_CHECK(f.x_range().min < inside.min - 1e-6);
+
+  // ... and the release arriving out there still ends it.
+  const plot::Range outside = f.x_range();
+  f.frame(ImVec2(470.0f, 470.0f), false);
+  PLOT_CHECK(!f.panning());
+  f.frame(ImVec2(430.0f, 430.0f), false);
+  PLOT_CHECK_NEAR(f.x_range().min, outside.min, 1e-9);
+}
+
+PLOT_TEST(imgui_widget_wheel_zooms_when_hovered_and_does_not_scroll_the_host) {
+  Fixture f;
+  f.tall_parent = true;  // the hosting window can scroll, so containment is testable
+  if (!f.init()) PLOT_CHECK(false);
+
+  // Control: the wheel scrolls the hosting window when it is over the window's own
+  // area. Downwards, because ImGui clamps a scroll at the top and would prove
+  // nothing. Two frames of quiet first: ImGui sizes a window's content (and so its
+  // scroll range) from the previous frame, and a wheel arriving before that range
+  // is known is not scrolled anywhere.
+  f.frame(ImVec2(470.0f, 470.0f), false);
+  f.frame(ImVec2(470.0f, 470.0f), false);
+  const float scroll_before = f.scroll_y;
+  f.frame(ImVec2(470.0f, 470.0f), false, -1.0f);
+  PLOT_CHECK(f.scroll_y > scroll_before + 1.0f);
+
+  // The same wheel over the parent must not zoom the plot: it is not hovered.
+  const plot::Range over_parent = f.x_range();
+  f.frame(ImVec2(470.0f, 470.0f), false, -1.0f);
+  PLOT_CHECK_NEAR(f.x_range().span(), over_parent.span(), 1e-9);
+
+  // Over the plot, the wheel zooms it (positive scrolls up, which zooms in) and
+  // the hosting window does not scroll.
+  f.frame(ImVec2(200.0f, 200.0f), false);  // hover: claims the wheel for the plot
+  const float scroll_before_plot = f.scroll_y;
+  const double span_before = f.x_range().span();
+  f.frame(ImVec2(200.0f, 200.0f), false, -1.0f);
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  PLOT_CHECK(f.x_range().span() > span_before);
+  PLOT_CHECK_NEAR(static_cast<double>(f.scroll_y), static_cast<double>(scroll_before_plot), 1e-3);
+
+  const double span_before_in = f.x_range().span();
+  f.frame(ImVec2(200.0f, 200.0f), false, 1.0f);
+  PLOT_CHECK(f.x_range().span() < span_before_in);
+}
+
+PLOT_TEST(imgui_widget_invalid_position_does_not_move_the_range) {
+  Fixture f;
+  if (!f.init()) PLOT_CHECK(false);
+
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.frame(ImVec2(200.0f, 200.0f), true);  // press over the plot: capture
+  const plot::Range pressed = f.x_range();
+
+  // The platform backends leave (-FLT_MAX, -FLT_MAX) behind when the pointer leaves
+  // the window (imgui_impl_glfw's cursor-enter callback) or the window loses focus
+  // (ClearInputMouse). Scaled by the framebuffer scale and differenced against the
+  // last real position, that is a ~1e35 movement -- and with a press captured, a
+  // ~1e35 pan, which `set_x_range` would take (and, clearing auto scale, would not
+  // give back).
+  f.frame(ImVec2(-kMaxFloat, -kMaxFloat), true);
+  PLOT_CHECK(std::isfinite(f.x_range().min) && std::isfinite(f.x_range().max));
+  PLOT_CHECK_NEAR(f.x_range().min, pressed.min, 1e-9);
+  PLOT_CHECK_NEAR(f.x_range().max, pressed.max, 1e-9);
+  PLOT_CHECK(f.panning());  // still captured: only the position was unusable
+
+  // The release still ends the drag even with no usable position in the event.
+  f.frame(ImVec2(-kMaxFloat, -kMaxFloat), false);
+  PLOT_CHECK(!f.panning());
+
+  // And a real position after the sentinel is not differenced against the sentinel.
+  f.frame(ImVec2(300.0f, 300.0f), false);
+  PLOT_CHECK_NEAR(f.x_range().min, pressed.min, 1e-9);
+  PLOT_CHECK_NEAR(f.x_range().max, pressed.max, 1e-9);
+}
+
+PLOT_TEST(imgui_widget_paints_the_whole_child_when_nothing_clips_it) {
+  Fixture f;
+  if (!f.init()) PLOT_CHECK(false);
+  f.clear_host();
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.scan_painted();
+
+  // The requested child rectangle, in top-left coordinates.
+  const u32 x0 = static_cast<u32>(f.child_origin.x);
+  const u32 y0 = static_cast<u32>(f.child_origin.y);
+  const u32 x1 = x0 + static_cast<u32>(f.child_w);
+  const u32 y1 = y0 + static_cast<u32>(f.child_h);
+
+  // The clamp against the clip rect must be a no-op when nothing clips the child:
+  // the painted rectangle is exactly the child's, corner to corner.
+  PLOT_CHECK(f.paint_min_x <= x0 + 1u && f.paint_min_y <= y0 + 1u);
+  PLOT_CHECK(f.paint_max_x >= x1 - 2u && f.paint_max_y >= y1 - 2u);
+
+  // ... and nothing of it escapes: the pixels past the child's edge are still the
+  // host's.
+  std::uint8_t out[4] = {0, 0, 0, 0};
+  f.pixel(x1 + 6u, y1 + 6u, out);
+  PLOT_CHECK(f.is_host(out));
+  f.pixel(x1 + 6u, y0 + 6u, out);
+  PLOT_CHECK(f.is_host(out));
+}
+
+PLOT_TEST(imgui_widget_rebinding_a_plot_does_not_fabricate_a_press) {
+  Fixture f;
+  if (!f.init()) PLOT_CHECK(false);
+
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.frame(ImVec2(200.0f, 200.0f), true);  // press over the first plot
+  PLOT_CHECK(f.panning());
+
+  // The host switches to its second plot with the button still held. ImGui reports
+  // a *level*, so there is no press edge in this frame: synthesising one out of the
+  // clear state a re-bind starts from would begin a drag the user never made.
+  f.frame(ImVec2(210.0f, 200.0f), true, 0.0f, &f.other);
+  PLOT_CHECK(f.widget.controller() != nullptr);
+  PLOT_CHECK(!f.panning());
+  const plot::Range other_before = f.other.x_range(f.ax_other);
+  f.frame(ImVec2(260.0f, 200.0f), true, 0.0f, &f.other);
+  PLOT_CHECK(!f.panning());
+  PLOT_CHECK_NEAR(f.other.x_range(f.ax_other).min, other_before.min, 1e-9);
+  f.frame(ImVec2(260.0f, 200.0f), false, 0.0f, &f.other);  // release
+  PLOT_CHECK(!f.panning());
+
+  // Back on the first plot, a press that *is* a press still starts a drag: the
+  // button state was kept across the re-bind, not frozen.
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.frame(ImVec2(200.0f, 200.0f), true);
+  PLOT_CHECK(f.panning());
+}
+
+PLOT_TEST(imgui_widget_paints_only_the_visible_part_of_a_clipped_child) {
+  Fixture f;
+  f.win_w = 200.0f;  // the child (400x400) cannot fit, so ImGui clips it
+  f.win_h = 200.0f;
+  if (!f.init()) PLOT_CHECK(false);
+  f.clear_host();
+  // The pointer is outside the window entirely: this is about painting, not input.
+  f.frame(ImVec2(300.0f, 300.0f), false);
+  f.scan_painted();
+
+  const u32 x0 = static_cast<u32>(f.child_origin.x);
+  const u32 y0 = static_cast<u32>(f.child_origin.y);
+  const u32 x1 = x0 + static_cast<u32>(f.child_w);  // the *requested* rectangle
+  const u32 y1 = y0 + static_cast<u32>(f.child_h);
+
+  // GL draws do not go through ImGui's clipping, so a child the window cuts off
+  // would otherwise clear and paint over the host's pixels beside it. The painted
+  // rectangle must stop well short of the requested one ...
+  PLOT_CHECK(f.paint_max_x + 20u < x1);
+  PLOT_CHECK(f.paint_max_y + 20u < y1);
+  // ... while still covering the part that is visible: this is a clamp, not a
+  // refusal to draw.
+  PLOT_CHECK(f.paint_max_x - f.paint_min_x > 100u);
+  PLOT_CHECK(f.paint_max_y - f.paint_min_y > 100u);
+
+  // The pixel deep inside the requested rectangle but outside the visible part kept
+  // the host's colour, and the one near the visible corner was painted.
+  std::uint8_t out[4] = {0, 0, 0, 0};
+  f.pixel(x1 - 5u, y1 - 5u, out);
+  PLOT_CHECK(f.is_host(out));
+  f.pixel(f.paint_min_x + 40u, f.paint_min_y + 40u, out);
+  PLOT_CHECK(!f.is_host(out));
+}