Found the problem file.

NOW TO DEBUG

exp_test.go

@@ -290,9 +290,9 @@ func TestSTUFF(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	os.Remove(wd + "/testing/test.txt")
-	testOut, _ := os.Create(wd + "/testing/test.txt")
-	testFil, err := rdr.Open("PortableApps/Notepad++Portable/App/Readme.txt")
+	os.Remove(wd + "/testing/test.xml")
+	testOut, _ := os.Create(wd + "/testing/test.xml")
+	testFil, err := rdr.Open("PortableApps/Notepad++Portable/App/Notepad++64/functionList/cobol-free.xml")
 	if err != nil {
 		t.Fatal(err)
 	}

internal/metadata/reader.go

@@ -2,6 +2,8 @@ package metadata
 
 import (
 	"encoding/binary"
+	"errors"
+	"fmt"
 	"io"
 
 	"github.com/CalebQ42/squashfs/internal/decompress"
@@ -36,6 +38,10 @@ func (r *Reader) Advance() error {
 	}
 	comp := size&0x8000 != 0x8000
 	size &^= 0x8000
+	if size > 8196 {
+		fmt.Println("uhoh")
+		return errors.New("AH")
+	}
 	r.cur = io.LimitReader(r.master, int64(size))
 	if comp {
 		r.cur, err = r.d.Reader(r.cur)

squashfs_test.go

@@ -72,8 +72,8 @@ func TestExtractQuick(t *testing.T) {
 	}
 	op := squashfs.DefaultOptions()
 	op.Verbose = true
-	sub := 
-	err = rdr.ExtractWithOptions(libPath, op)
+	sub, _ := rdr.Sub("PortableApps/Notepad++Portable/App/Notepad++64/functionList")
+	err = sub.(*squashfs.FS).ExtractWithOptions(libPath, op)
 	if err != nil {
 		t.Fatal(err)
 	}