Drag and Drop: Disable tracking mouse button ownership when an external drag source is active, to make it easier to achieve drag and drop over multiple OS windows. (#143)

imgui.cpp

@@ -2456,8 +2456,9 @@ void ImGui::NewFrame()
     g.OsImePosRequest = ImVec2(1.0f, 1.0f); // OS Input Method Editor showing on top-left of our window by default
 
     // If mouse was first clicked outside of ImGui bounds we also cancel out hovering.
-    // FIXME: For patterns of drag and drop between "application" and "imgui" we may need to rework/remove this test (first committed 311c0ca9 on 2015/02)
-    if (!mouse_avail_to_imgui)
+    // FIXME: For patterns of drag and drop across OS windows, we may need to rework/remove this test (first committed 311c0ca9 on 2015/02)
+    bool mouse_dragging_extern_payload = g.DragDropActive && (g.DragDropSourceFlags & ImGuiDragDropFlags_SourceExtern) != 0;
+    if (!mouse_avail_to_imgui && !mouse_dragging_extern_payload)
         g.HoveredWindow = g.HoveredRootWindow = NULL;
 
     // Scale & Scrolling

imgui_demo.cpp

@@ -1734,7 +1734,10 @@ void ImGui::ShowTestWindow(bool* p_open)
 
         if (ImGui::TreeNode("Keyboard & Mouse State"))
         {
-            ImGui::Text("Mouse pos: (%g, %g)", io.MousePos.x, io.MousePos.y);
+            if (ImGui::IsMousePosValid())
+                ImGui::Text("Mouse pos: (%g, %g)", io.MousePos.x, io.MousePos.y);
+            else
+                ImGui::Text("Mouse pos: <INVALID>");
             ImGui::Text("Mouse down:");     for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++) if (io.MouseDownDuration[i] >= 0.0f)   { ImGui::SameLine(); ImGui::Text("b%d (%.02f secs)", i, io.MouseDownDuration[i]); }
             ImGui::Text("Mouse clicked:");  for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++) if (ImGui::IsMouseClicked(i))          { ImGui::SameLine(); ImGui::Text("b%d", i); }
             ImGui::Text("Mouse dbl-clicked:"); for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++) if (ImGui::IsMouseDoubleClicked(i)) { ImGui::SameLine(); ImGui::Text("b%d", i); }