diff --git a/juce_amalgamated.cpp b/juce_amalgamated.cpp
index b44613d1ce..66aee53a0e 100644
--- a/juce_amalgamated.cpp
+++ b/juce_amalgamated.cpp
@@ -22668,7 +22668,12 @@ void AudioThumbnail::setLevels (const MinMaxValue* const* values, int thumbIndex
 	for (int i = jmin (numChans, channels.size()); --i >= 0;)
 		channels.getUnchecked(i)->write (values[i], thumbIndex, numValues);
 
-	numSamplesFinished = jmax (numSamplesFinished, (thumbIndex + numValues) * (int64) samplesPerThumbSample);
+	const int64 start = thumbIndex * (int64) samplesPerThumbSample;
+	const int64 end = (thumbIndex + numValues) * (int64) samplesPerThumbSample;
+
+	if (numSamplesFinished >= start && end > numSamplesFinished)
+		numSamplesFinished = end;
+
 	totalSamples = jmax (numSamplesFinished, totalSamples);
 	window->invalidate();
 	sendChangeMessage();
@@ -244735,8 +244740,8 @@ void FileOutputStream::flushInternal()
 
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
 	: address (nullptr),
-	  internal (nullptr),
-	  length (0)
+	  length (0),
+	  fileHandle (nullptr)
 {
 	jassert (mode == readOnly || mode == readWrite);
 
@@ -244754,10 +244759,9 @@ MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMo
 	HANDLE h = CreateFile (file.getFullPathName().toWideCharPointer(), accessMode, FILE_SHARE_READ, 0,
 						   createType, FILE_ATTRIBUTE_NORMAL, 0);
 
-	internal = (void*) h;
-
 	if (h != INVALID_HANDLE_VALUE)
 	{
+		fileHandle = (void*) h;
 		const int64 fileSize = file.getSize();
 
 		HANDLE mappingHandle = CreateFileMapping (h, 0, protect, (DWORD) (fileSize >> 32), (DWORD) fileSize, 0);
@@ -244778,8 +244782,8 @@ MemoryMappedFile::~MemoryMappedFile()
 	if (address != nullptr)
 		UnmapViewOfFile (address);
 
-	if (internal != nullptr)
-		CloseHandle ((HANDLE) internal);
+	if (fileHandle != nullptr)
+		CloseHandle ((HANDLE) fileHandle);
 }
 
 int64 File::getSize() const
@@ -260572,42 +260576,37 @@ void FileOutputStream::flushInternal()
 
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
 	: address (nullptr),
-	  internal (nullptr),
-	  length (0)
+	  length (0),
+	  fileHandle (0)
 {
 	jassert (mode == readOnly || mode == readWrite);
 
-	const int fd = open (file.getFullPathName().toUTF8(),
-						 mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
+	fileHandle = open (file.getFullPathName().toUTF8(),
+					   mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
 
-	internal = reinterpret_cast<void*> (fd);
-
-	if (fd != -1)
+	if (fileHandle != -1)
 	{
 		const int64 fileSize = file.getSize();
 
-		address = mmap (0, (size_t) fileSize,
+		void* m = mmap (0, (size_t) fileSize,
 						mode == readWrite ? (PROT_READ | PROT_WRITE) : PROT_READ,
-						MAP_SHARED, fd, 0);
+						MAP_SHARED, fileHandle, 0);
 
-		if (address == MAP_FAILED)
-			address = nullptr;
-		else
+		if (m != MAP_FAILED)
+		{
+			address = m;
 			length = (size_t) fileSize;
+		}
 	}
 }
 
 MemoryMappedFile::~MemoryMappedFile()
 {
-	const int fd = reinterpret_cast <int> (internal);
-
-	if (fd != 0)
-	{
-		if (address != nullptr)
-			munmap (address, length);
+	if (address != nullptr)
+		munmap (address, length);
 
-		close (fd);
-	}
+	if (fileHandle != 0)
+		close (fileHandle);
 }
 
 const File juce_getExecutableFile()
@@ -270603,42 +270602,37 @@ void FileOutputStream::flushInternal()
 
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
 	: address (nullptr),
-	  internal (nullptr),
-	  length (0)
+	  length (0),
+	  fileHandle (0)
 {
 	jassert (mode == readOnly || mode == readWrite);
 
-	const int fd = open (file.getFullPathName().toUTF8(),
-						 mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
-
-	internal = reinterpret_cast<void*> (fd);
+	fileHandle = open (file.getFullPathName().toUTF8(),
+					   mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
 
-	if (fd != -1)
+	if (fileHandle != -1)
 	{
 		const int64 fileSize = file.getSize();
 
-		address = mmap (0, (size_t) fileSize,
+		void* m = mmap (0, (size_t) fileSize,
 						mode == readWrite ? (PROT_READ | PROT_WRITE) : PROT_READ,
-						MAP_SHARED, fd, 0);
+						MAP_SHARED, fileHandle, 0);
 
-		if (address == MAP_FAILED)
-			address = nullptr;
-		else
+		if (m != MAP_FAILED)
+		{
+			address = m;
 			length = (size_t) fileSize;
+		}
 	}
 }
 
 MemoryMappedFile::~MemoryMappedFile()
 {
-	const int fd = reinterpret_cast <int> (internal);
-
-	if (fd != 0)
-	{
-		if (address != nullptr)
-			munmap (address, length);
+	if (address != nullptr)
+		munmap (address, length);
 
-		close (fd);
-	}
+	if (fileHandle != 0)
+		close (fileHandle);
 }
 
 const File juce_getExecutableFile()
@@ -287469,42 +287463,37 @@ void FileOutputStream::flushInternal()
 
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
 	: address (nullptr),
-	  internal (nullptr),
-	  length (0)
+	  length (0),
+	  fileHandle (0)
 {
 	jassert (mode == readOnly || mode == readWrite);
 
-	const int fd = open (file.getFullPathName().toUTF8(),
-						 mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
+	fileHandle = open (file.getFullPathName().toUTF8(),
+					   mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
 
-	internal = reinterpret_cast<void*> (fd);
-
-	if (fd != -1)
+	if (fileHandle != -1)
 	{
 		const int64 fileSize = file.getSize();
 
-		address = mmap (0, (size_t) fileSize,
+		void* m = mmap (0, (size_t) fileSize,
 						mode == readWrite ? (PROT_READ | PROT_WRITE) : PROT_READ,
-						MAP_SHARED, fd, 0);
+						MAP_SHARED, fileHandle, 0);
 
-		if (address == MAP_FAILED)
-			address = nullptr;
-		else
+		if (m != MAP_FAILED)
+		{
+			address = m;
 			length = (size_t) fileSize;
+		}
 	}
 }
 
 MemoryMappedFile::~MemoryMappedFile()
 {
-	const int fd = reinterpret_cast <int> (internal);
-
-	if (fd != 0)
-	{
-		if (address != nullptr)
-			munmap (address, length);
+	if (address != nullptr)
+		munmap (address, length);
 
-		close (fd);
-	}
+	if (fileHandle != 0)
+		close (fileHandle);
 }
 
 const File juce_getExecutableFile()
diff --git a/juce_amalgamated.h b/juce_amalgamated.h
index c901c56b7f..0f5ab51cf0 100644
--- a/juce_amalgamated.h
+++ b/juce_amalgamated.h
@@ -19463,9 +19463,14 @@ public:
 private:
 
 	void* address;
-	void* internal;
 	size_t length;
 
+   #if JUCE_WINDOWS
+	void* fileHandle;
+   #else
+	int fileHandle;
+   #endif
+
 	JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (MemoryMappedFile);
 };
 
diff --git a/src/audio/audio_file_formats/juce_AudioThumbnail.cpp b/src/audio/audio_file_formats/juce_AudioThumbnail.cpp
index fc055ad564..1f727eada9 100644
--- a/src/audio/audio_file_formats/juce_AudioThumbnail.cpp
+++ b/src/audio/audio_file_formats/juce_AudioThumbnail.cpp
@@ -706,7 +706,12 @@ void AudioThumbnail::setLevels (const MinMaxValue* const* values, int thumbIndex
     for (int i = jmin (numChans, channels.size()); --i >= 0;)
         channels.getUnchecked(i)->write (values[i], thumbIndex, numValues);
 
-    numSamplesFinished = jmax (numSamplesFinished, (thumbIndex + numValues) * (int64) samplesPerThumbSample);
+    const int64 start = thumbIndex * (int64) samplesPerThumbSample;
+    const int64 end = (thumbIndex + numValues) * (int64) samplesPerThumbSample;
+
+    if (numSamplesFinished >= start && end > numSamplesFinished)
+        numSamplesFinished = end;
+
     totalSamples = jmax (numSamplesFinished, totalSamples);
     window->invalidate();
     sendChangeMessage();
diff --git a/src/io/files/juce_MemoryMappedFile.h b/src/io/files/juce_MemoryMappedFile.h
index 140bfb2ab3..210d9e87b4 100644
--- a/src/io/files/juce_MemoryMappedFile.h
+++ b/src/io/files/juce_MemoryMappedFile.h
@@ -74,9 +74,14 @@ public:
 private:
     //==============================================================================
     void* address;
-    void* internal;
     size_t length;
 
+   #if JUCE_WINDOWS
+    void* fileHandle;
+   #else
+    int fileHandle;
+   #endif
+
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (MemoryMappedFile);
 };
 
diff --git a/src/native/common/juce_posix_SharedCode.h b/src/native/common/juce_posix_SharedCode.h
index c9d4a9bda2..1bb268f4a8 100644
--- a/src/native/common/juce_posix_SharedCode.h
+++ b/src/native/common/juce_posix_SharedCode.h
@@ -526,42 +526,37 @@ void FileOutputStream::flushInternal()
 //==============================================================================
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
     : address (nullptr),
-      internal (nullptr),
-      length (0)
+      length (0),
+      fileHandle (0)
 {
     jassert (mode == readOnly || mode == readWrite);
 
-    const int fd = open (file.getFullPathName().toUTF8(),
-                         mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
+    fileHandle = open (file.getFullPathName().toUTF8(),
+                       mode == readWrite ? (O_CREAT + O_RDWR) : O_RDONLY, 00644);
 
-    internal = reinterpret_cast<void*> (fd);
-
-    if (fd != -1)
+    if (fileHandle != -1)
     {
         const int64 fileSize = file.getSize();
 
-        address = mmap (0, (size_t) fileSize,
+        void* m = mmap (0, (size_t) fileSize,
                         mode == readWrite ? (PROT_READ | PROT_WRITE) : PROT_READ,
-                        MAP_SHARED, fd, 0);
+                        MAP_SHARED, fileHandle, 0);
 
-        if (address == MAP_FAILED)
-            address = nullptr;
-        else
+        if (m != MAP_FAILED)
+        {
+            address = m;
             length = (size_t) fileSize;
+        }
     }
 }
 
 MemoryMappedFile::~MemoryMappedFile()
 {
-    const int fd = reinterpret_cast <int> (internal);
+    if (address != nullptr)
+        munmap (address, length);
 
-    if (fd != 0)
-    {
-        if (address != nullptr)
-            munmap (address, length);
-
-        close (fd);
-    }
+    if (fileHandle != 0)
+        close (fileHandle);
 }
 
 //==============================================================================
diff --git a/src/native/windows/juce_win32_Files.cpp b/src/native/windows/juce_win32_Files.cpp
index 67637a50e8..76c58845b4 100644
--- a/src/native/windows/juce_win32_Files.cpp
+++ b/src/native/windows/juce_win32_Files.cpp
@@ -302,8 +302,8 @@ void FileOutputStream::flushInternal()
 //==============================================================================
 MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMode mode)
     : address (nullptr),
-      internal (nullptr),
-      length (0)
+      length (0),
+      fileHandle (nullptr)
 {
     jassert (mode == readOnly || mode == readWrite);
 
@@ -321,10 +321,9 @@ MemoryMappedFile::MemoryMappedFile (const File& file, MemoryMappedFile::AccessMo
     HANDLE h = CreateFile (file.getFullPathName().toWideCharPointer(), accessMode, FILE_SHARE_READ, 0,
                            createType, FILE_ATTRIBUTE_NORMAL, 0);
 
-    internal = (void*) h;
-
     if (h != INVALID_HANDLE_VALUE)
     {
+        fileHandle = (void*) h;
         const int64 fileSize = file.getSize();
 
         HANDLE mappingHandle = CreateFileMapping (h, 0, protect, (DWORD) (fileSize >> 32), (DWORD) fileSize, 0);
@@ -345,8 +344,8 @@ MemoryMappedFile::~MemoryMappedFile()
     if (address != nullptr)
         UnmapViewOfFile (address);
 
-    if (internal != nullptr)
-        CloseHandle ((HANDLE) internal);
+    if (fileHandle != nullptr)
+        CloseHandle ((HANDLE) fileHandle);
 }
 
 //==============================================================================