Fix trackpad scroll bounce in ImGui panels

src/slic3r/GUI/ImGuiWrapper.cpp

@@ -505,10 +505,12 @@ bool ImGuiWrapper::update_mouse_data(wxMouseEvent& evt)
     io.MouseDown[0] = evt.LeftIsDown();
     io.MouseDown[1] = evt.RightIsDown();
     io.MouseDown[2] = evt.MiddleIsDown();
-    float wheel_delta = static_cast<float>(evt.GetWheelDelta());
-    if (wheel_delta != 0.0f) {
+    // Only a vertical wheel event with a direction is a wheel step. A macOS trackpad reports sideways motion
+    // as separate horizontal events, and its wheel events often carry a rotation of 0: wx truncates the
+    // sub-pixel deltas to whole ones and passes on events with no delta at all. Read as vertical steps,
+    // either kind scrolls against the gesture, and scrolling up bounces back and forth.
+    if (evt.GetWheelAxis() == wxMOUSE_WHEEL_VERTICAL && evt.GetWheelRotation() != 0)
         io.MouseWheel = evt.GetWheelRotation() > 0 ? 1.f : -1.f;
-    }
     unsigned buttons = (evt.LeftIsDown() ? 1 : 0) | (evt.RightIsDown() ? 2 : 0) | (evt.MiddleIsDown() ? 4 : 0);
     m_mouse_buttons = buttons;
 

tests/slic3rutils/CMakeLists.txt

@@ -43,6 +43,7 @@ add_executable(${_TEST_NAME}_tests
     test_texture_color_palette.cpp
     test_texture_island_view.cpp
     test_file_url.cpp
+    test_imgui_wrapper.cpp
     test_user_manager.cpp
     ../fff_print/test_helpers.cpp
     )

tests/slic3rutils/test_imgui_wrapper.cpp

@@ -0,0 +1,61 @@
+// How ImGuiWrapper turns wx mouse events into ImGui input. ImGuiWrapper lives in libslic3r_gui; this is
+// the suite that links it. Same Windows include prologue as test_texture_color_palette.cpp (wx pulls in
+// <windows.h>; keep WIN32_LEAN_AND_MEAN / NOMINMAX ahead of it).
+#ifdef WIN32
+    #ifndef WIN32_LEAN_AND_MEAN
+        #define WIN32_LEAN_AND_MEAN
+    #endif
+    #ifndef NOMINMAX
+        #define NOMINMAX
+    #endif
+    #include <Windows.h>
+#endif
+
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/generators/catch_generators.hpp>
+#include <catch2/matchers/catch_matchers.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+
+#include <imgui/imgui.h>
+#include <wx/event.h>
+
+#include "slic3r/GUI/ImGuiWrapper.hpp"
+
+using Catch::Matchers::WithinAbs;
+using Slic3r::GUI::ImGuiWrapper;
+
+namespace {
+
+// ImGui's wheel after one wheel event, as wx on macOS builds it: a delta of 10 and the rotation in whole
+// points of trackpad travel.
+float wheel_after(int rotation, wxMouseWheelAxis axis)
+{
+    ImGuiWrapper imgui;
+    imgui.set_display_size(800.f, 600.f); // update_mouse_data() ignores input until the display has a size
+
+    wxMouseEvent evt(wxEVT_MOUSEWHEEL);
+    evt.m_wheelRotation = rotation;
+    evt.m_wheelDelta    = 10;
+    evt.m_wheelAxis     = axis;
+    imgui.update_mouse_data(evt);
+    return ImGui::GetIO().MouseWheel;
+}
+
+} // namespace
+
+TEST_CASE("a vertical wheel event steps ImGui's wheel in its own direction", "[ImGuiWrapper]")
+{
+    const int rotation = GENERATE(1, 7, 120, -1, -7, -120);
+    CHECK_THAT(wheel_after(rotation, wxMOUSE_WHEEL_VERTICAL), WithinAbs(rotation > 0 ? 1.0 : -1.0, 0.0));
+}
+
+// Both kinds arrive from a macOS trackpad while scrolling; see ImGuiWrapper::update_mouse_data().
+TEST_CASE("a wheel event with no rotation or on the horizontal axis does not step ImGui's wheel", "[ImGuiWrapper][Regression]")
+{
+    const auto [rotation, axis] = GENERATE(table<int, wxMouseWheelAxis>({
+        { 0, wxMOUSE_WHEEL_VERTICAL },
+        { 5, wxMOUSE_WHEEL_HORIZONTAL },
+        { -5, wxMOUSE_WHEEL_HORIZONTAL },
+    }));
+    CHECK_THAT(wheel_after(rotation, axis), WithinAbs(0.0, 0.0));
+}