diff --git a/internal/lsp/source/format.go b/internal/lsp/source/format.go
index 5823a6e..09a903f 100644
--- a/internal/lsp/source/format.go
+++ b/internal/lsp/source/format.go
@@ -192,8 +192,9 @@
 	return ToProtocolEdits(origMapper, edits)
 }
 
-// return the prefix of the src through the last imports, or if there are
-// no imports, through the package statement (and a subsequent comment group)
+// importPrefix returns the prefix of the given file content through the final
+// import statement. If there are no imports, the prefix is the package
+// statement and any comment groups below it.
 func importPrefix(src []byte) string {
 	fset := token.NewFileSet()
 	// do as little parsing as possible
@@ -201,28 +202,54 @@
 	if err != nil { // This can happen if 'package' is misspelled
 		return ""
 	}
-	myStart := fset.File(f.Pos()).Base() // 1, but the generality costs little
-	pkgEnd := int(f.Name.NamePos) + len(f.Name.Name)
+	tok := fset.File(f.Pos())
 	var importEnd int
 	for _, d := range f.Decls {
 		if x, ok := d.(*ast.GenDecl); ok && x.Tok == token.IMPORT {
-			e := int(d.End()) - myStart
-			if e > importEnd {
+			if e := tok.Offset(d.End()); e > importEnd {
 				importEnd = e
 			}
 		}
 	}
-	if importEnd == 0 {
-		importEnd = pkgEnd
-		if importEnd > len(src) {
-			importEnd-- // pkgEnd is off by 1 because Pos is 1-based
+
+	maybeAdjustToLineEnd := func(pos token.Pos, isCommentNode bool) int {
+		offset := tok.Offset(pos)
+
+		// Don't go past the end of the file.
+		if offset > len(src) {
+			offset = len(src)
 		}
+		// The go/ast package does not account for different line endings, and
+		// specifically, in the text of a comment, it will strip out \r\n line
+		// endings in favor of \n. To account for these differences, we try to
+		// return a position on the next line whenever possible.
+		switch line := tok.Line(tok.Pos(offset)); {
+		case line < tok.LineCount():
+			nextLineOffset := tok.Offset(tok.LineStart(line + 1))
+			// If we found a position that is at the end of a line, move the
+			// offset to the start of the next line.
+			if offset+1 == nextLineOffset {
+				offset = nextLineOffset
+			}
+		case isCommentNode, offset+1 == tok.Size():
+			// If the last line of the file is a comment, or we are at the end
+			// of the file, the prefix is the entire file.
+			offset = len(src)
+		}
+		return offset
+	}
+	if importEnd == 0 {
+		pkgEnd := f.Name.End()
+		importEnd = maybeAdjustToLineEnd(pkgEnd, false)
 	}
 	for _, c := range f.Comments {
-		if int(c.End()) > importEnd {
-			importEnd = int(c.End())
+		if end := tok.Offset(c.End()); end > importEnd {
+			importEnd = maybeAdjustToLineEnd(c.End(), true)
 		}
 	}
+	if importEnd > len(src) {
+		importEnd = len(src)
+	}
 	return string(src[:importEnd])
 }
 
diff --git a/internal/lsp/source/format_test.go b/internal/lsp/source/format_test.go
index f929812..5030806 100644
--- a/internal/lsp/source/format_test.go
+++ b/internal/lsp/source/format_test.go
@@ -1,15 +1,17 @@
 package source
 
 import (
+	"fmt"
 	"testing"
+
+	"golang.org/x/tools/internal/lsp/diff"
+	"golang.org/x/tools/internal/lsp/diff/myers"
 )
 
-type data struct {
-	input, want string
-}
-
 func TestImportPrefix(t *testing.T) {
-	var tdata = []data{
+	for i, tt := range []struct {
+		input, want string
+	}{
 		{"package foo", "package foo"},
 		{"package foo\n", "package foo\n"},
 		{"package foo\n\nfunc f(){}\n", "package foo\n"},
@@ -19,13 +21,29 @@
 		{"// hi \n\npackage foo //xx\nfunc _(){}\n", "// hi \n\npackage foo //xx\n"},
 		{"package foo //hi\n", "package foo //hi\n"},
 		{"//hi\npackage foo\n//a\n\n//b\n", "//hi\npackage foo\n//a\n\n//b\n"},
-		{"package a\n\nimport (\n  \"fmt\"\n)\n//hi\n",
-			"package a\n\nimport (\n  \"fmt\"\n)\n//hi\n"},
-	}
-	for i, x := range tdata {
-		got := importPrefix([]byte(x.input))
-		if got != x.want {
-			t.Errorf("%d: got\n%q, wanted\n%q for %q", i, got, x.want, x.input)
+		{
+			"package a\n\nimport (\n  \"fmt\"\n)\n//hi\n",
+			"package a\n\nimport (\n  \"fmt\"\n)\n//hi\n",
+		},
+		{`package a /*hi*/`, `package a /*hi*/`},
+		{"package main\r\n\r\nimport \"go/types\"\r\n\r\n/*\r\n\r\n */\r\n", "package main\r\n\r\nimport \"go/types\"\r\n\r\n/*\r\n\r\n */\r\n"},
+		{"package x; import \"os\"; func f() {}\n\n", "package x; import \"os\""},
+		{"package x; func f() {fmt.Println()}\n\n", "package x"},
+	} {
+		got := importPrefix([]byte(tt.input))
+		if got != tt.want {
+			t.Errorf("%d: failed for %q:\n%s", i, tt.input, diffStr(tt.want, got))
 		}
 	}
 }
+
+func diffStr(want, got string) string {
+	if want == got {
+		return ""
+	}
+	// Add newlines to avoid newline messages in diff.
+	want += "\n"
+	got += "\n"
+	d := myers.ComputeEdits("", want, got)
+	return fmt.Sprintf("%q", diff.ToUnified("want", "got", want, d))
+}