font/opentype: report valid bounds from layoutText for the empty string

Signed-off-by: Elias Naur <mail@eliasnaur.com>

font/opentype/opentype.go

@@ -176,9 +176,10 @@ func layoutText(sbuf *sfnt.Buffer, ppem fixed.Int26_6, maxWidth int, fonts []*op
 	}
 	var prev, word state
 	endLine := func() {
-		if prev.f != nil {
-			updateBounds(prev.f)
+		if prev.f == nil && len(fonts) > 0 {
+			prev.f = fonts[0]
 		}
+		updateBounds(prev.f)
 		nextLine.Layout = glyphs[:prev.idx:prev.idx]
 		nextLine.Len = prev.len
 		nextLine.Width = prev.x + prev.adv

font/opentype/opentype_test.go

@@ -13,6 +13,9 @@ import (
 	"gioui.org/internal/ops"
 	"gioui.org/op"
 	"gioui.org/text"
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/gofont/goregular"
+	"golang.org/x/image/font/sfnt"
 	"golang.org/x/image/math/fixed"
 )
 
@@ -87,6 +90,31 @@ func TestCollectionAsFace(t *testing.T) {
 	}
 }
 
+func TestEmptyString(t *testing.T) {
+	face, err := Parse(goregular.TTF)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ppem := fixed.I(200)
+
+	lines, err := face.Layout(ppem, 2000, strings.NewReader(""))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(lines) == 0 {
+		t.Fatalf("Layout returned no lines for empty string; expected 1")
+	}
+	l := lines[0]
+	exp, err := face.font.Bounds(new(sfnt.Buffer), ppem, font.HintingFull)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := l.Bounds; got != exp {
+		t.Errorf("got bounds %+v for empty string; expected %+v", got, exp)
+	}
+}
+
 func decompressFontFile(name string) (*Font, []byte, error) {
 	f, err := os.Open(name)
 	if err != nil {