ImGuiIntegration: ensure too fast mouse clicks aren't ignored.

If both a press and a release happened in the same frame, ImGui wouldn't
know about them.

Co-authored-by: Marco Melorio <m.melorio@icloud.com>

doc/changelog-integration.dox

@@ -60,6 +60,10 @@ namespace Magnum {
 -   @ref ImGuiIntegration::Context::handleKeyPressEvent() "ImGuiIntegration::Context::handleKey*Event()"
     now handles the @m_class{m-label m-default} **Space** key as well (see
     [mosra/magnum-integration#44](https://github.com/mosra/magnum-integration/pull/44))
+-   @ref ImGuiIntegration::Context::handleMousePressEvent() and
+    @ref ImGuiIntegration::Context::handleMouseReleaseEvent() are fixed to
+    avoid ignoring mouse clicks if both press and release happens in the same
+    frame
 -   Assorted @ref DartIntegration updates (see
     [mosra/magnum-integration#47](https://github.com/mosra/magnum-integration/pull/47)):
     -   @ref DartIntegration::convertShapeNode() can now handle cones as well

src/Magnum/ImGuiIntegration/Context.cpp

@@ -248,9 +248,20 @@ void Context::newFrame() {
 
     _timeline.nextFrame();
 
-    ImGui::GetIO().DeltaTime = _timeline.previousFrameDuration();
+    ImGuiIO& io = ImGui::GetIO();
+    io.DeltaTime = _timeline.previousFrameDuration();
+
+    /* Fire delayed mouse events. This sets MouseDown both in case the press
+       happened in this frame but also if both press and release happened at
+       the same frame */
+    for(const Int buttonId: {0, 1, 2})
+        io.MouseDown[buttonId] = _mousePressed[buttonId] || _mousePressedInThisFrame[buttonId];
 
     ImGui::NewFrame();
+
+    /* It's a new frame, clear any indicators for received mouse presses in
+       this frame */
+    _mousePressedInThisFrame = {};
 }
 
 void Context::drawFrame() {

src/Magnum/ImGuiIntegration/Context.h

@@ -510,6 +510,7 @@ class MAGNUM_IMGUIINTEGRATION_EXPORT Context {
         GL::Mesh _mesh;
         Vector2 _supersamplingRatio,
             _eventScaling;
+        BoolVector3 _mousePressed, _mousePressedInThisFrame;
 
     private:
         template<class KeyEvent> bool handleKeyEvent(KeyEvent& event, bool value);

src/Magnum/ImGuiIntegration/Context.hpp

@@ -140,21 +140,30 @@ template<class MouseEvent> bool Context::handleMouseEvent(MouseEvent& event, boo
     ImGuiIO& io = ImGui::GetIO();
     io.MousePos = ImVec2(Vector2(event.position())*_eventScaling);
 
+    std::size_t buttonId;
     switch(event.button()) {
         case MouseEvent::Button::Left:
-            io.MouseDown[0] = value;
+            buttonId = 0;
             break;
         case MouseEvent::Button::Right:
-            io.MouseDown[1] = value;
+            buttonId = 1;
             break;
         case MouseEvent::Button::Middle:
-            io.MouseDown[2] = value;
+            buttonId = 2;
             break;
 
         /* Unknown button, do nothing */
         default: return false;
     }
 
+    /* Instead of setting io.MouseDown directly, we delay this until the
+       newFrame() call in order to prevent mouse clicks from being ignored when
+       both a press and a release happens in the same frame. Apart from this
+       happening when the app can't render fast enough, for some reason it also
+       happens with SDL2 on macOS -- press delayed by a significant  */
+    _mousePressed.set(buttonId, value);
+    if(value) _mousePressedInThisFrame.set(buttonId, true);
+
     return io.WantCaptureMouse;
 }
 

src/Magnum/ImGuiIntegration/Test/ContextGLTest.cpp

@@ -125,6 +125,7 @@ struct ContextGLTest: GL::OpenGLTester {
     void relayoutRefreshFonts();
 
     void mouseInput();
+    void mouseInputTooFast();
     void keyInput();
     void textInput();
 
@@ -149,6 +150,7 @@ ContextGLTest::ContextGLTest() {
               &ContextGLTest::relayoutRefreshFonts,
 
               &ContextGLTest::mouseInput,
+              &ContextGLTest::mouseInputTooFast,
               &ContextGLTest::keyInput,
               &ContextGLTest::textInput,
 
@@ -477,29 +479,52 @@ void ContextGLTest::mouseInput() {
     MouseEvent right{Button::Right, {3, 4}, {}};
     MouseEvent middle{Button::Middle, {5, 6}, {}};
 
+    /* In order to avoid imgui ignoring mouse clicks if both press and release
+       happens in the same frame, the events are propagated to it only during
+       newFrame() -- which means we need to check *after* it gets called. See
+       mouseInputTooFast() below for more. */
+
     c.handleMousePressEvent(left);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(ImGui::IsMouseDown(0)); /* left */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{1.0f, 2.0f}));
+    c.drawFrame();
 
     c.handleMousePressEvent(right);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(ImGui::IsMouseDown(1)); /* right */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{3.0f, 4.0f}));
+    c.drawFrame();
 
     c.handleMousePressEvent(middle);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(ImGui::IsMouseDown(2)); /* middle */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{5.0f, 6.0f}));
+    c.drawFrame();
 
     c.handleMouseReleaseEvent(right);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(!ImGui::IsMouseDown(1)); /* right */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{3.0f, 4.0f}));
+    c.drawFrame();
 
     c.handleMouseReleaseEvent(left);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(!ImGui::IsMouseDown(0)); /* left */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{1.0f, 2.0f}));
+    c.drawFrame();
 
     c.handleMouseReleaseEvent(middle);
+    Utility::System::sleep(1);
+    c.newFrame();
     CORRADE_VERIFY(!ImGui::IsMouseDown(2)); /* middle */
     CORRADE_COMPARE(Vector2(ImGui::GetMousePos()), (Vector2{5.0f, 6.0f}));
+    c.drawFrame();
 
     /* Mouse Movement */
     MouseEvent move{Button{}, {1, 2}, {}};
@@ -522,6 +547,28 @@ void ContextGLTest::mouseInput() {
     CORRADE_VERIFY(!c.handleMouseReleaseEvent(unknownButton));
 }
 
+void ContextGLTest::mouseInputTooFast() {
+    Context c{{200, 200}};
+
+    CORRADE_VERIFY(!ImGui::IsMouseDown(0));
+
+    /* It's not reported immediately */
+    MouseEvent left{Button::Left, {1, 2}, {}};
+    c.handleMousePressEvent(left);
+    c.handleMouseReleaseEvent(left);
+    CORRADE_VERIFY(!ImGui::IsMouseDown(0));
+
+    /* Only during the newFrame call */
+    c.newFrame();
+    CORRADE_VERIFY(ImGui::IsMouseDown(0));
+    c.drawFrame();
+
+    /* And mouse up is reported in the next one */
+    c.newFrame();
+    CORRADE_VERIFY(!ImGui::IsMouseDown(0));
+    c.drawFrame();
+}
+
 void ContextGLTest::keyInput() {
     Context c{{}};