2 files changed, 74 insertions(+), 3 deletions(-)

M text/shaper.go
M text/shaper_test.go

@@ 247,9 247,11 @@ func (l *Shaper) layoutText(params Parameters, txt io.Reader, str string) {
 					break
 				}
 			}
-			_, re := l.reader.ReadByte()
-			done = re != nil
-			_ = l.reader.UnreadByte()
+			if !done {
+				_, re := l.reader.ReadByte()
+				done = re != nil
+				_ = l.reader.UnreadByte()
+			}
 		} else {
 			idx := strings.IndexByte(str, '\n')
 			if idx == -1 {

@@ 6,6 6,7 @@ import (
 	"testing"
 
 	nsareg "eliasnaur.com/font/noto/sans/arabic/regular"
+	"gioui.org/font/gofont"
 	"gioui.org/font/opentype"
 	"gioui.org/io/system"
 	"golang.org/x/exp/slices"


@@ 431,3 432,71 @@ func printLinePositioning(t *testing.T, lines []line, glyphs []Glyph) {
 		}
 	}
 }
+
+// TestShapeStringRuneAccounting tries shaping the same string/parameter combinations with both
+// shaping methods and ensures that the resulting glyph stream always has the right number of
+// runes accounted for.
+func TestShapeStringRuneAccounting(t *testing.T) {
+	type testcase struct {
+		name   string
+		input  string
+		params Parameters
+	}
+	type setup struct {
+		kind string
+		do   func(*Shaper, Parameters, string)
+	}
+	for _, tc := range []testcase{
+		{
+			name:  "simple truncated",
+			input: "abc",
+			params: Parameters{
+				PxPerEm:  fixed.Int26_6(16),
+				MaxWidth: 100,
+				MaxLines: 1,
+			},
+		},
+		{
+			name:  "simple",
+			input: "abc",
+			params: Parameters{
+				PxPerEm:  fixed.Int26_6(16),
+				MaxWidth: 100,
+			},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			for _, setup := range []setup{
+				{
+					kind: "LayoutString",
+					do: func(shaper *Shaper, params Parameters, input string) {
+						shaper.LayoutString(params, input)
+					},
+				},
+				{
+					kind: "Layout",
+					do: func(shaper *Shaper, params Parameters, input string) {
+						shaper.Layout(params, strings.NewReader(input))
+					},
+				},
+			} {
+				t.Run(setup.kind, func(t *testing.T) {
+					shaper := NewShaper(gofont.Collection())
+					setup.do(shaper, tc.params, tc.input)
+
+					glyphs := []Glyph{}
+					for g, ok := shaper.NextGlyph(); ok; g, ok = shaper.NextGlyph() {
+						glyphs = append(glyphs, g)
+					}
+					totalRunes := 0
+					for _, g := range glyphs {
+						totalRunes += g.Runes
+					}
+					if inputRunes := len([]rune(tc.input)); totalRunes != inputRunes {
+						t.Errorf("input contained %d runes, but glyphs contained %d", inputRunes, totalRunes)
+					}
+				})
+			}
+		})
+	}
+}