Backends: Added support for SDL 2.0.18+ preciseX/preciseY mouse wheel data for smooth scrolling + scaling for Emscripten. (#4019, #6096)

+ Missing changelog entries.

backends/imgui_impl_sdl.cpp

@@ -18,6 +18,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2023-02-02: Added support for SDL 2.0.18+ preciseX/preciseY mouse wheel data for smooth scrolling + Scaling X value on Emscripten (bug?). (#4019, #6096)
 //  2023-02-02: Removed SDL_MOUSEWHEEL value clamping, as values seem correct in latest Emscripten. (#4019)
 //  2023-02-01: Flipping SDL_MOUSEWHEEL 'wheel.x' value to match other backends and offer consistent horizontal scrolling direction. (#4019, #6096, #1463)
 //  2022-10-11: Using 'nullptr' instead of 'NULL' as per our switch to C++11.
@@ -262,8 +263,17 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
         }
         case SDL_MOUSEWHEEL:
         {
+            //IMGUI_DEBUG_LOG("wheel %.2f %.2f, precise %.2f %.2f\n", (float)event->wheel.x, (float)event->wheel.y, event->wheel.preciseX, event->wheel.preciseY);
+#if SDL_VERSION_ATLEAST(2,0,18) // If this fails to compile on Emscripten: update to latest Emscripten!
+            float wheel_x = -event->wheel.preciseX;
+            float wheel_y = event->wheel.preciseY;
+#else
             float wheel_x = -(float)event->wheel.x;
             float wheel_y = (float)event->wheel.y;
+#endif
+#ifdef __EMSCRIPTEN__
+            wheel_x /= 100.0f;
+#endif
             io.AddMouseWheelEvent(wheel_x, wheel_y);
             return true;
         }