Avoid pool reallocation in `BasicJsonDocument`'s copy assignment

extras/tests/JsonDocument/BasicJsonDocument.cpp

@@ -89,7 +89,7 @@ TEST_CASE("BasicJsonDocument") {
   }
 #endif
 
-  SECTION("Copy assign") {
+  SECTION("Copy assign larger") {
     {
       BasicJsonDocument<SpyingAllocator> doc1(4096, log);
       doc1.set(std::string("The size of this string is 32!!"));
@@ -104,6 +104,36 @@ TEST_CASE("BasicJsonDocument") {
     REQUIRE(log.str() == "A4096A8FA4096FF");
   }
 
+  SECTION("Copy assign smaller") {
+    {
+      BasicJsonDocument<SpyingAllocator> doc1(1024, log);
+      doc1.set(std::string("The size of this string is 32!!"));
+      BasicJsonDocument<SpyingAllocator> doc2(4096, log);
+
+      doc2 = doc1;
+
+      REQUIRE(doc1.as<std::string>() == "The size of this string is 32!!");
+      REQUIRE(doc2.as<std::string>() == "The size of this string is 32!!");
+      REQUIRE(doc2.capacity() == 1024);
+    }
+    REQUIRE(log.str() == "A1024A4096FA1024FF");
+  }
+
+  SECTION("Copy assign same size") {
+    {
+      BasicJsonDocument<SpyingAllocator> doc1(1024, log);
+      doc1.set(std::string("The size of this string is 32!!"));
+      BasicJsonDocument<SpyingAllocator> doc2(1024, log);
+
+      doc2 = doc1;
+
+      REQUIRE(doc1.as<std::string>() == "The size of this string is 32!!");
+      REQUIRE(doc2.as<std::string>() == "The size of this string is 32!!");
+      REQUIRE(doc2.capacity() == 1024);
+    }
+    REQUIRE(log.str() == "A1024A1024FF");
+  }
+
 #if ARDUINOJSON_HAS_RVALUE_REFERENCES
   SECTION("Move assign") {
     {

src/ArduinoJson/Document/BasicJsonDocument.hpp

@@ -139,6 +139,9 @@ class BasicJsonDocument : AllocatorOwner<TAllocator>, public JsonDocument {
   }
 
   void reallocPool(size_t requiredSize) {
+    size_t capa = addPadding(requiredSize);
+    if (capa == _pool.capacity())
+      return;
     freePool();
     replacePool(allocPool(addPadding(requiredSize)));
   }