Swallow a diff renderer's protocol handshake

Before the diff, a conforming renderer emits one OSC 1717 record that
carries only the version — its way of announcing that it speaks the
protocol at all, without a host having to inspect what it renders. It
describes no line, so keeping it would give the first line of the diff a
record that says nothing about it.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/gocui/escape.go

@@ -489,8 +489,10 @@ func (ei *escapeInterpreter) parseOne(ch []byte) (isEscape bool, err error) {
 	case stateOSCMetadata:
 		switch {
 		case characterEquals(ch, 0x07):
+			ei.dropMetadataIfHandshake()
 			ei.state = stateNone
 		case characterEquals(ch, 0x1b):
+			ei.dropMetadataIfHandshake()
 			ei.state = stateOSCEndEscape
 		default:
 			ei.metadata.Write(ch)
@@ -511,6 +513,18 @@ func (ei *escapeInterpreter) parseOne(ch []byte) (isEscape bool, err error) {
 	return false, nil
 }
 
+// dropMetadataIfHandshake discards a just-completed OSC 1717 payload that
+// carries nothing beyond the version. A diff renderer emits such a record ahead
+// of everything else to announce that it speaks the protocol, so that a host can
+// find that out by asking rather than by inspecting a rendering. It says nothing
+// about any line, so it must not attach to the line that follows it; a per-line
+// record always has fields, and is kept.
+func (ei *escapeInterpreter) dropMetadataIfHandshake() {
+	if !strings.Contains(ei.metadata.String(), ";") {
+		ei.metadata.Reset()
+	}
+}
+
 func (ei *escapeInterpreter) outputCSI() error {
 	n := len(ei.csiParam)
 	for i := 0; i < n; {

pkg/gocui/view_test.go

@@ -314,6 +314,23 @@ func TestDiffLineContentsOfWrappedLine(t *testing.T) {
 	}
 }
 
+func TestDiffLineContentsSwallowsHandshake(t *testing.T) {
+	v := NewView("name", 0, 0, 80, 10, OutputNormal)
+
+	// A diff renderer announces itself with a version-only record before the
+	// diff. It must leave no trace: no visible bytes, no line of its own, and
+	// above all no record on the line that follows it.
+	v.writeString(osc1717("1") + strings.Join([]string{
+		"diff --git a/foo.txt b/foo.txt",
+		osc1717("1;a;1;;foo.txt") + "added",
+	}, "\n"))
+
+	assert.Equal(t, []DiffLineContent{
+		{Text: "diff --git a/foo.txt b/foo.txt"},
+		{Text: "added", Metadata: []string{"1;a;1;;foo.txt"}},
+	}, v.DiffLineContents())
+}
+
 // An async re-render builds into an off-screen buffer and swaps it in once it
 // has enough to paint, so readers keep seeing the previous render — coherent and
 // consistent — until the new content appears in one step. See View.offscreen.