diff --git a/lib/private/Files/Stream/Quota.php b/lib/private/Files/Stream/Quota.php
index 3c7e09c633bc7..79d651d400edb 100644
--- a/lib/private/Files/Stream/Quota.php
+++ b/lib/private/Files/Stream/Quota.php
@@ -48,40 +48,59 @@ public function dir_opendir($path, $options) {
 	}
 
 	public function stream_seek($offset, $whence = SEEK_SET) {
-		if ($whence === SEEK_END) {
-			// go to the end to find out last position's offset
-			$oldOffset = $this->stream_tell();
-			if (fseek($this->source, 0, $whence) !== 0) {
-				return false;
-			}
-			$whence = SEEK_SET;
-			$offset = $this->stream_tell() + $offset;
-			$this->limit += $oldOffset - $offset;
-		} elseif ($whence === SEEK_SET) {
-			$this->limit += $this->stream_tell() - $offset;
-		} else {
-			$this->limit -= $offset;
-		}
 		// this wrapper needs to return "true" for success.
 		// the fseek call itself returns 0 on succeess
 		return fseek($this->source, $offset, $whence) === 0;
 	}
 
 	public function stream_read($count) {
-		$this->limit -= $count;
+		// Do not decrement $this->limit for reads
 		return fread($this->source, $count);
 	}
 
 	public function stream_write($data) {
 		$size = strlen($data);
-		if ($size > $this->limit) {
-			$data = substr($data, 0, $this->limit);
-			$size = $this->limit;
+
+		// Get current pointer and file size
+		$curPos = ftell($this->source);
+		fseek($this->source, 0, SEEK_END);
+		$fileSize = ftell($this->source);
+		fseek($this->source, $curPos, SEEK_SET);
+
+		$writeEnd = $curPos + $size;
+
+		// Calculate how many bytes are "new" (beyond end of existing)
+		$newBytes = max(0, $writeEnd - $fileSize);
+
+		// Enforce quota for new bytes only
+		if ($newBytes > $this->limit) {
+			 // Only this many new bytes are permitted:
+			$allowedNewBytes = $this->limit;
+			// Adjust write size to fit quota
+			// Calculate max amount we can write, given cursor position and allowed new bytes
+			$allowedSize = $size - ($newBytes - $allowedNewBytes);
+
+			if ($allowedSize <= 0) {
+				return 0; // No new bytes allowed
+			}
+			$data = substr($data, 0, $allowedSize);
+			$size = $allowedSize;
+			// Recalculate position/write end after truncation (for safety)
+			$writeEnd = $curPos + $size;
+			$newBytes = max(0, $writeEnd - $fileSize);
 		}
+
 		$written = fwrite($this->source, $data);
+
+		// Decrement limit by actually written new bytes
+		// (Extra safety: recalculate actual new bytes in case fwrite was truncated)
+		$actualWriteEnd = ftell($this->source);
+		$actualFileSize = max($fileSize, $actualWriteEnd);
+		$actualNewBytes = max(0, $actualFileSize - $fileSize);
 		// Decrement quota by the actual number of bytes written ($written),
 		// not the intended size
-		$this->limit -= $written;
+		$this->limit -= $actualNewBytes;
+
 		return $written;
 	}
 }
diff --git a/tests/lib/Files/Stream/QuotaTest.php b/tests/lib/Files/Stream/QuotaTest.php
index 4248d14f5a1fe..c1ddcd1546d13 100644
--- a/tests/lib/Files/Stream/QuotaTest.php
+++ b/tests/lib/Files/Stream/QuotaTest.php
@@ -28,6 +28,42 @@ public function testWriteEnoughSpace(): void {
 		$this->assertEquals('foobar', fread($stream, 100));
 	}
 
+	// Quota is not reduced by reads or seeks
+	public function testReadsAndSeeksDoNotAffectQuota() {
+		$stream = $this->getStream('w+', 6);
+		$this->assertEquals('', fread($stream, 3)); // Read before write
+		$this->assertEquals(6, fwrite($stream, 'foobar')); // Write to fill quota
+		rewind($stream);
+		$this->assertEquals('foo', fread($stream, 3));
+		fseek($stream, 1, SEEK_CUR); // Seek should not affect quota
+		$this->assertEquals(0, fwrite($stream, 'baz')); // No quota left for growth
+		rewind($stream);
+		$this->assertEquals('foobar', fread($stream, 100));
+	}
+
+	// Overwriting after quota is exhausted should succeed
+	public function testOverwriteAfterQuotaExhausted() {
+		$stream = $this->getStream('w+', 3);
+		fwrite($stream, 'abc'); // Fill quota
+		$this->assertEquals(0, fwrite($stream, 'd')); // No quota for growth
+		rewind($stream);
+		$this->assertEquals(3, fwrite($stream, 'xyz')); // Overwrite: should succeed
+		rewind($stream);
+		$this->assertEquals('xyz', fread($stream, 100));
+	}
+
+	// Quota is only used for file growth
+	public function testQuotaOnlyUsedForGrowth() {
+		$stream = $this->getStream('w+', 5);
+		$this->assertEquals(5, fwrite($stream, '12345')); // Fill quota
+		rewind($stream);
+		$this->assertEquals(3, fwrite($stream, 'abc')); // Overwrite allowed
+		fseek($stream, 5, SEEK_SET);
+		$this->assertEquals(0, fwrite($stream, 'xx')); // No quota for extension
+		rewind($stream);
+		$this->assertEquals('abc45', fread($stream, 100));
+	}
+
 	public function testWriteNotEnoughSpace(): void {
 		$stream = $this->getStream('w+', 3);
 		$this->assertEquals(3, fwrite($stream, 'foobar'));