Internals: ensure ButtonBehavior() is called with non 0 id.

imgui_widgets.cpp

@@ -509,7 +509,7 @@ bool ImGui::ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool
 
 #ifdef IMGUI_ENABLE_TEST_ENGINE
     // Alternate registration spot, for when caller didn't use ItemAdd()
-    if (id != 0 && g.LastItemData.ID != id)
+    if (g.LastItemData.ID != id)
         IMGUI_TEST_ENGINE_ITEM_ADD(id, bb, NULL);
 #endif
 
@@ -537,6 +537,8 @@ bool ImGui::ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool
     const ImGuiID test_owner_id = (flags & ImGuiButtonFlags_NoTestKeyOwner) ? ImGuiKeyOwner_Any : id;
     if (hovered)
     {
+        IM_ASSERT(id != 0); // Lazily check inside rare path.
+
         // Poll mouse buttons
         // - 'mouse_button_clicked' is generally carried into ActiveIdMouseButton when setting ActiveId.
         // - Technically we only need some values in one code path, but since this is gated by hovered test this is fine.