Overlap: moved ImGuiItemflags_AllowOverlap handling from ButtoBehavior() to ItemHoverable() now that it is possible. (#6512, #3909, #517)

This allows DragXXX, SliderXXX, PlotXXX etc to honor SetNextItemAllowOverlap().

imgui.cpp

@@ -4069,6 +4069,15 @@ bool ImGui::ItemHoverable(const ImRect& bb, ImGuiID id, ImGuiItemFlags item_flag
             return false;
 
         SetHoveredID(id);
+
+        // AllowOverlap mode (rarely used) requires previous frame HoveredId to be null or to match.
+        // This allows using patterns where a later submitted widget overlaps a previous one. Generally perceived as a front-to-back hit-test.
+        if (item_flags & ImGuiItemflags_AllowOverlap)
+        {
+            g.HoveredIdAllowOverlap = true;
+            if (g.HoveredIdPreviousFrame != id)
+                return false;
+        }
     }
 
     // When disabled we'll return false but still set HoveredId