Basic undocumented/unsupported combination of Child+Tooltip. The full feature needs substancially more work but this is enough for simplest cases. (#1345)

imgui.cpp

@@ -4560,10 +4560,12 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     // When reusing window again multiple times a frame, just append content (don't need to setup again)
     if (first_begin_of_the_frame)
     {
+        const bool is_pinned_child_tooltip = (flags & ImGuiWindowFlags_ChildWindow) && (flags & ImGuiWindowFlags_Tooltip); // FIXME-WIP: Undocumented behavior of Child+Tooltip for pinned tooltip (#1345)
+
         // Initialize
         window->ParentWindow = parent_window;
         window->RootWindow = window->RootNonPopupWindow = window;
-        if (parent_window && (flags & ImGuiWindowFlags_ChildWindow))
+        if (parent_window && (flags & ImGuiWindowFlags_ChildWindow) && !is_pinned_child_tooltip)
             window->RootWindow = parent_window->RootWindow;
         if (parent_window && !(flags & ImGuiWindowFlags_Modal) && (flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_Popup)))
             window->RootNonPopupWindow = parent_window->RootNonPopupWindow;
@@ -4681,8 +4683,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         {
             window->BeginOrderWithinParent = parent_window->DC.ChildWindows.Size;
             parent_window->DC.ChildWindows.push_back(window);
-
+            if (!(flags & ImGuiWindowFlags_Popup) && !window_pos_set_by_api && !is_pinned_child_tooltip)
-            if (!(flags & ImGuiWindowFlags_Popup) && !window_pos_set_by_api)
                 window->Pos = window->PosFloat = parent_window->DC.CursorPos;
         }
 
@@ -4713,7 +4714,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         }
 
         // Position tooltip (always follows mouse)
-        if ((flags & ImGuiWindowFlags_Tooltip) != 0 && !window_pos_set_by_api)
+        if ((flags & ImGuiWindowFlags_Tooltip) != 0 && !window_pos_set_by_api && !is_pinned_child_tooltip)
         {
             ImVec2 ref_pos = g.IO.MousePos;
             ImRect rect_to_avoid(ref_pos.x - 16, ref_pos.y - 8, ref_pos.x + 24, ref_pos.y + 24); // FIXME: Completely hard-coded. Store boxes in mouse cursor data? Scale? Center on cursor hit-point?
@@ -4771,7 +4772,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->DrawList->Flags = (g.Style.AntiAliasedLines ? ImDrawListFlags_AntiAliasedLines : 0) | (g.Style.AntiAliasedFill ? ImDrawListFlags_AntiAliasedFill : 0);
         window->DrawList->PushTextureID(g.Font->ContainerAtlas->TexID);
         ImRect fullscreen_rect(GetVisibleRect());
-        if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup))
+        if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup) && !is_pinned_child_tooltip)
             PushClipRect(parent_window->ClipRect.Min, parent_window->ClipRect.Max, true);
         else
             PushClipRect(fullscreen_rect.Min, fullscreen_rect.Max, true);