CollapsingHeader() with close button adapt to horizontal scrolling (#600)

imgui.cpp

@@ -5958,7 +5958,7 @@ bool ImGui::CollapsingHeader(const char* label, bool* p_open, ImGuiTreeNodeFlags
         // Create a small overlapping close button // FIXME: We can evolve this into user accessible helpers to add extra buttons on title bars, headers, etc.
         ImGuiContext& g = *GImGui;
         float button_sz = g.FontSize * 0.5f;
-        if (CloseButton(window->GetID((void*)(intptr_t)(id+1)), ImVec2(window->DC.LastItemRect.Max.x - g.Style.FramePadding.x - button_sz, window->DC.LastItemRect.Min.y + g.Style.FramePadding.y + button_sz), button_sz))
+        if (CloseButton(window->GetID((void*)(intptr_t)(id+1)), ImVec2(ImMin(window->DC.LastItemRect.Max.x, window->ClipRect.Max.x) - g.Style.FramePadding.x - button_sz, window->DC.LastItemRect.Min.y + g.Style.FramePadding.y + button_sz), button_sz))
             *p_open = false;
     }