Tables: fixed calculation of multi-instance shared decoration/scrollbar width of scrolling tables. (#5920, #6619)

Avoid width variation when resizing down a table hosting a child window.
+ shallow tweak to GetContentRegionMax().

imgui_tables.cpp

@@ -410,7 +410,7 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
             table->HasScrollbarYPrev = table->HasScrollbarYCurr;
             table->HasScrollbarYCurr = false;
         }
-        table->HasScrollbarYCurr |= (table->InnerWindow->ScrollMax.y > 0.0f);
+        table->HasScrollbarYCurr |= table->InnerWindow->ScrollbarY;
     }
     else
     {