Using the ImGuiWindowFlags_NoScrollWithMouse flag on a child window forwards the mouse wheel event to the parent window, unless either ImGuiWindowFlags_NoInputs or ImGuiWindowFlags_NoScrollbar are also set. (#1380, #1502)

imgui.cpp

@@ -213,6 +213,7 @@
  Here is a change-log of API breaking changes, if you are using one of the functions listed, expect to have to fix some code.
  Also read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2017/12/14 (1.53) - using the ImGuiWindowFlags_NoScrollWithMouse flag on a child window forwards the mouse wheel event to the parent window, unless either ImGuiWindowFlags_NoInputs or ImGuiWindowFlags_NoScrollbar are also set.
  - 2017/12/13 (1.53) - renamed GetItemsLineHeightWithSpacing() to GetFrameHeightWithSpacing(). Kept redirection function (will obsolete).
  - 2017/12/13 (1.53) - obsoleted IsRootWindowFocused() in favor of using IsWindowFocused(ImGuiFocusedFlags_RootWindow). Kept redirection function (will obsolete).
                      - obsoleted IsRootWindowOrAnyChildFocused() in favor of using IsWindowFocused(ImGuiFocusedFlags_RootAndChildWindows). Kept redirection function (will obsolete).
@@ -2462,12 +2463,20 @@ void ImGui::NewFrame()
             window->Size *= scale;
             window->SizeFull *= scale;
         }
-        else if (!g.IO.KeyCtrl && !(window->Flags & ImGuiWindowFlags_NoScrollWithMouse))
+        else if (!g.IO.KeyCtrl)
         {
             // Mouse wheel Scrolling
-            float scroll_amount = 5 * window->CalcFontSize();
-            scroll_amount = (float)(int)ImMin(scroll_amount, (window->ContentsRegionRect.GetHeight() + window->WindowPadding.y * 2.0f) * 0.67f);
-            SetWindowScrollY(window, window->Scroll.y - g.IO.MouseWheel * scroll_amount);
+            // If a child window has the ImGuiWindowFlags_NoScrollWithMouse flag, we give a chance to scroll its parent (unless either ImGuiWindowFlags_NoInputs or ImGuiWindowFlags_NoScrollbar are also set).
+            ImGuiWindow* scroll_window = window;
+            while ((scroll_window->Flags & ImGuiWindowFlags_ChildWindow) && (scroll_window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(scroll_window->Flags & ImGuiWindowFlags_NoScrollbar) && !(scroll_window->Flags & ImGuiWindowFlags_NoInputs) && scroll_window->ParentWindow)
+                scroll_window = scroll_window->ParentWindow;
+
+            if (!(scroll_window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(scroll_window->Flags & ImGuiWindowFlags_NoInputs))
+            {
+                float scroll_amount = 5 * scroll_window->CalcFontSize();
+                scroll_amount = (float)(int)ImMin(scroll_amount, (scroll_window->ContentsRegionRect.GetHeight() + scroll_window->WindowPadding.y * 2.0f) * 0.67f);
+                SetWindowScrollY(scroll_window, scroll_window->Scroll.y - g.IO.MouseWheel * scroll_amount);
+            }
         }
     }
 

imgui.h

@@ -524,7 +524,7 @@ enum ImGuiWindowFlags_
     ImGuiWindowFlags_NoResize               = 1 << 1,   // Disable user resizing with the lower-right grip
     ImGuiWindowFlags_NoMove                 = 1 << 2,   // Disable user moving the window
     ImGuiWindowFlags_NoScrollbar            = 1 << 3,   // Disable scrollbars (window can still scroll with mouse or programatically)
-    ImGuiWindowFlags_NoScrollWithMouse      = 1 << 4,   // Disable user vertically scrolling with mouse wheel
+    ImGuiWindowFlags_NoScrollWithMouse      = 1 << 4,   // Disable user vertically scrolling with mouse wheel. On child window, mouse wheel will be forwarded to the parent unless NoScrollbar is also set.
     ImGuiWindowFlags_NoCollapse             = 1 << 5,   // Disable user collapsing window by double-clicking on it
     ImGuiWindowFlags_AlwaysAutoResize       = 1 << 6,   // Resize every window to its content every frame
     //ImGuiWindowFlags_ShowBorders          = 1 << 7,   // Show borders around windows and items (OBSOLETE! Use e.g. style.FrameBorderSize=1.0f to enable borders).

imgui_demo.cpp

@@ -997,6 +997,9 @@ void ImGui::ShowTestWindow(bool* p_open)
     {
         if (ImGui::TreeNode("Child regions"))
         {
+            static bool disable_mouse_wheel = false;
+            ImGui::Checkbox("Disable Mouse Wheel", &disable_mouse_wheel);
+
             ImGui::Text("Without border");
             static int line = 50;
             bool goto_line = ImGui::Button("Goto");
@@ -1004,7 +1007,8 @@ void ImGui::ShowTestWindow(bool* p_open)
             ImGui::PushItemWidth(100);
             goto_line |= ImGui::InputInt("##Line", &line, 0, 0, ImGuiInputTextFlags_EnterReturnsTrue);
             ImGui::PopItemWidth();
-            ImGui::BeginChild("Sub1", ImVec2(ImGui::GetWindowContentRegionWidth() * 0.5f,300), false, ImGuiWindowFlags_HorizontalScrollbar);
+
+            ImGui::BeginChild("Sub1", ImVec2(ImGui::GetWindowContentRegionWidth() * 0.5f,300), false, ImGuiWindowFlags_HorizontalScrollbar | (disable_mouse_wheel ? ImGuiWindowFlags_NoScrollWithMouse : 0));
             for (int i = 0; i < 100; i++)
             {
                 ImGui::Text("%04d: scrollable region", i);
@@ -1018,7 +1022,7 @@ void ImGui::ShowTestWindow(bool* p_open)
             ImGui::SameLine();
 
             ImGui::PushStyleVar(ImGuiStyleVar_ChildRounding, 5.0f);
-            ImGui::BeginChild("Sub2", ImVec2(0,300), true);
+            ImGui::BeginChild("Sub2", ImVec2(0,300), true, (disable_mouse_wheel ? ImGuiWindowFlags_NoScrollWithMouse : 0));
             ImGui::Text("With border");
             ImGui::Columns(2);
             for (int i = 0; i < 100; i++)