Break preview lines on IND to support chafa in preview window on tmux

chafa ends each row of a Kitty Unicode placeholder image with CUB + IND
instead of a newline, so that an image drawn at a column offset survives
ONLCR, which would rewrite the newline as CR NL and pull the cursor back
to column 0. Both sequences were dropped, so the image arrived as a single
line and was re-wrapped, shifting every row after the first by the width
of the wrap sign. Without wrapping it was truncated to its first row.

The column is tracked across the breaks and restored with padding, so an
image indented by the preview command keeps its indent.

Fix #4885

src/terminal_test.go

@@ -851,3 +851,73 @@ func TestWordWrapAnsiLine(t *testing.T) {
 		t.Errorf("Tab wrap: %q", result)
 	}
 }
+
+func TestSplitOnIND(t *testing.T) {
+	term := &Terminal{tabstop: 8}
+	for _, tc := range []struct {
+		name string
+		line string
+		want []string
+	}{
+		{
+			// Nothing to do, so the caller keeps the line as it read it
+			name: "no IND",
+			line: "foo\n",
+			want: nil,
+		},
+		{
+			// chafa: CUB returns to the column the row started on
+			name: "rows at column 0",
+			line: "AAA\x1b[3D\x1bDBBB\x1b[3D\x1bDCCC\n",
+			want: []string{"AAA\x1b[3D\n", "BBB\x1b[3D\n", "CCC\n"},
+		},
+		{
+			// 'printf "  "; chafa ...'
+			name: "indented rows",
+			line: "  AAA\x1b[3D\x1bDBBB\x1b[3D\x1bDCCC\n",
+			want: []string{"  AAA\x1b[3D\n", "  BBB\x1b[3D\n", "  CCC\n"},
+		},
+		{
+			name: "tab indent expands to the tab stop",
+			line: "\tAAA\x1b[3D\x1bDBBB\x1b[3D\x1bDCCC\n",
+			want: []string{"\tAAA\x1b[3D\n", "        BBB\x1b[3D\n", "        CCC\n"},
+		},
+		{
+			// IND on its own keeps the column
+			name: "bare IND",
+			line: "AB\x1bDCD\n",
+			want: []string{"AB\n", "  CD\n"},
+		},
+		{
+			name: "CUB without a parameter moves back one",
+			line: "AB\x1b[D\x1bDCD\n",
+			want: []string{"AB\x1b[D\n", " CD\n"},
+		},
+		{
+			name: "CUB past the left edge is clamped",
+			line: "AB\x1b[9D\x1bDCD\n",
+			want: []string{"AB\x1b[9D\n", "CD\n"},
+		},
+		{
+			name: "SGR codes take no column",
+			line: "\x1b[31mAB\x1b[m\x1b[2D\x1bDCD\n",
+			want: []string{"\x1b[31mAB\x1b[m\x1b[2D\n", "CD\n"},
+		},
+		{
+			name: "pass-throughs take no column",
+			line: "\x1b_Ga=T,c=2,r=1\x1b\\AB\x1b[2D\x1bDCD\n",
+			want: []string{"\x1b_Ga=T,c=2,r=1\x1b\\AB\x1b[2D\n", "CD\n"},
+		},
+	} {
+		got := term.splitOnIND(tc.line)
+		if len(got) != len(tc.want) {
+			t.Errorf("%s: got %q, want %q", tc.name, got, tc.want)
+			continue
+		}
+		for idx, line := range got {
+			if line != tc.want[idx] {
+				t.Errorf("%s: line %d: got %q, want %q", tc.name, idx, line, tc.want[idx])
+			}
+		}
+	}
+}