Correctly handle setting cells to zero values.

internal/canvas/buffer/buffer.go

@@ -115,6 +115,11 @@ func (b Buffer) SetCell(p image.Point, r rune, opts ...cell.Option) (int, error)
 		return -1, err
 	}
 	rw := runewidth.RuneWidth(r)
+	if rw == 0 {
+		// Even if the rune is invisible, like the zero-value rune, it still
+		// occupies at least the target cell.
+		rw = 1
+	}
 	if rw > remW {
 		return -1, fmt.Errorf("cannot set rune %q of width %d at point %v, only have %d remaining cells at this line", r, rw, p, remW)
 	}

internal/canvas/buffer/buffer_test.go

@@ -411,6 +411,18 @@ func TestSetCell(t *testing.T) {
 				return b
 			}(),
 		},
+		{
+			desc:      "sets zero-value rune in a cell",
+			buffer:    mustNew(image.Point{3, 3}),
+			point:     image.Point{1, 2},
+			r:         0,
+			wantCells: 1,
+			want: func() Buffer {
+				b := mustNew(size)
+				b[1][2].Rune = 0
+				return b
+			}(),
+		},
 		{
 			desc:   "sets cell options",
 			buffer: mustNew(image.Point{3, 3}),