diff --git a/src/libvterm/t/harness.c b/src/libvterm/t/harness.c
new file mode 100644
index 0000000..2b22464
--- /dev/null
+++ b/src/libvterm/t/harness.c
@@ -0,0 +1,929 @@
+#include "vterm.h"
+#include "../src/vterm_internal.h" /* We pull in some internal bits too */
+
+#include <stdio.h>
+#include <string.h>
+
+#define streq(a,b) (!strcmp(a,b))
+#define strstartswith(a,b) (!strncmp(a,b,strlen(b)))
+
+static size_t inplace_hex2bytes(char *s)
+{
+  char *inpos = s, *outpos = s;
+
+  while(*inpos) {
+    unsigned int ch;
+    sscanf(inpos, "%2x", &ch);
+    *outpos = ch;
+    outpos += 1; inpos += 2;
+  }
+
+  return outpos - s;
+}
+
+static VTermModifier strpe_modifiers(char **strp)
+{
+  VTermModifier state = 0;
+
+  while((*strp)[0]) {
+    switch(((*strp)++)[0]) {
+      case 'S': state |= VTERM_MOD_SHIFT; break;
+      case 'C': state |= VTERM_MOD_CTRL;  break;
+      case 'A': state |= VTERM_MOD_ALT;   break;
+      default: return state;
+    }
+  }
+
+  return state;
+}
+
+static VTermKey strp_key(char *str)
+{
+  static struct {
+    char *name;
+    VTermKey key;
+  } keys[] = {
+    { "Up",    VTERM_KEY_UP },
+    { "Tab",   VTERM_KEY_TAB },
+    { "Enter", VTERM_KEY_ENTER },
+    { "KP0",   VTERM_KEY_KP_0 },
+    { NULL,    VTERM_KEY_NONE },
+  };
+  int i;
+
+  for(i = 0; keys[i].name; i++) {
+    if(streq(str, keys[i].name))
+      return keys[i].key;
+  }
+
+  return VTERM_KEY_NONE;
+}
+
+static VTerm *vt;
+static VTermState *state;
+static VTermScreen *screen;
+
+static VTermEncodingInstance encoding;
+
+static int parser_text(const char bytes[], size_t len, void *user)
+{
+  int i;
+
+  printf("text ");
+  for(i = 0; i < len; i++) {
+    unsigned char b = bytes[i];
+    if(b < 0x20 || b == 0x7f || (b >= 0x80 && b < 0xa0))
+      break;
+    printf(i ? ",%x" : "%x", b);
+  }
+  printf("\n");
+
+  return i;
+}
+
+static int parser_control(unsigned char control, void *user)
+{
+  printf("control %02x\n", control);
+
+  return 1;
+}
+
+static int parser_escape(const char bytes[], size_t len, void *user)
+{
+  int i;
+
+  if(bytes[0] >= 0x20 && bytes[0] < 0x30) {
+    if(len < 2)
+      return -1;
+    len = 2;
+  }
+  else {
+    len = 1;
+  }
+
+  printf("escape ");
+  for(i = 0; i < len; i++)
+    printf("%02x", bytes[i]);
+  printf("\n");
+
+  return len;
+}
+
+static int parser_csi(const char *leader, const long args[], int argcount, const char *intermed, char command, void *user)
+{
+  int i;
+  printf("csi %02x", command);
+
+  if(leader && leader[0]) {
+    printf(" L=");
+    for(i = 0; leader[i]; i++)
+      printf("%02x", leader[i]);
+  }
+
+  for(i = 0; i < argcount; i++) {
+    char sep = i ? ',' : ' ';
+
+    if(args[i] == CSI_ARG_MISSING)
+      printf("%c*", sep);
+    else
+      printf("%c%ld%s", sep, CSI_ARG(args[i]), CSI_ARG_HAS_MORE(args[i]) ? "+" : "");
+  }
+
+  if(intermed && intermed[0]) {
+    printf(" I=");
+    for(i = 0; intermed[i]; i++)
+      printf("%02x", intermed[i]);
+  }
+
+  printf("\n");
+
+  return 1;
+}
+
+static int parser_osc(const char *command, size_t cmdlen, void *user)
+{
+  int i;
+  printf("osc ");
+  for(i = 0; i < cmdlen; i++)
+    printf("%02x", command[i]);
+  printf("\n");
+
+  return 1;
+}
+
+static int parser_dcs(const char *command, size_t cmdlen, void *user)
+{
+  int i;
+  printf("dcs ");
+  for(i = 0; i < cmdlen; i++)
+    printf("%02x", command[i]);
+  printf("\n");
+
+  return 1;
+}
+
+static VTermParserCallbacks parser_cbs = {
+  parser_text, /* text */
+  parser_control, /* control */
+  parser_escape, /* escape */
+  parser_csi, /* csi */
+  parser_osc, /* osc */
+  parser_dcs, /* dcs */
+  NULL /* resize */
+};
+
+/* These callbacks are shared by State and Screen */
+
+static int want_movecursor = 0;
+static VTermPos state_pos;
+static int movecursor(VTermPos pos, VTermPos oldpos, int visible, void *user)
+{
+  state_pos = pos;
+
+  if(want_movecursor)
+    printf("movecursor %d,%d\n", pos.row, pos.col);
+
+  return 1;
+}
+
+static int want_scrollrect = 0;
+static int scrollrect(VTermRect rect, int downward, int rightward, void *user)
+{
+  if(!want_scrollrect)
+    return 0;
+
+  printf("scrollrect %d..%d,%d..%d => %+d,%+d\n",
+      rect.start_row, rect.end_row, rect.start_col, rect.end_col,
+      downward, rightward);
+
+  return 1;
+}
+
+static int want_moverect = 0;
+static int moverect(VTermRect dest, VTermRect src, void *user)
+{
+  if(!want_moverect)
+    return 0;
+
+  printf("moverect %d..%d,%d..%d -> %d..%d,%d..%d\n",
+      src.start_row,  src.end_row,  src.start_col,  src.end_col,
+      dest.start_row, dest.end_row, dest.start_col, dest.end_col);
+
+  return 1;
+}
+
+static int want_settermprop = 0;
+static int settermprop(VTermProp prop, VTermValue *val, void *user)
+{
+  VTermValueType type;
+  if(!want_settermprop)
+    return 1;
+
+  type = vterm_get_prop_type(prop);
+  switch(type) {
+  case VTERM_VALUETYPE_BOOL:
+    printf("settermprop %d %s\n", prop, val->boolean ? "true" : "false");
+    return 1;
+  case VTERM_VALUETYPE_INT:
+    printf("settermprop %d %d\n", prop, val->number);
+    return 1;
+  case VTERM_VALUETYPE_STRING:
+    printf("settermprop %d \"%s\"\n", prop, val->string);
+    return 1;
+  case VTERM_VALUETYPE_COLOR:
+    printf("settermprop %d rgb(%d,%d,%d)\n", prop, val->color.red, val->color.green, val->color.blue);
+    return 1;
+  }
+
+  return 0;
+}
+
+/* These callbacks are for State */
+
+static int want_state_putglyph = 0;
+static int state_putglyph(VTermGlyphInfo *info, VTermPos pos, void *user)
+{
+  int i;
+  if(!want_state_putglyph)
+    return 1;
+
+  printf("putglyph ");
+  for(i = 0; info->chars[i]; i++)
+    printf(i ? ",%x" : "%x", info->chars[i]);
+  printf(" %d %d,%d", info->width, pos.row, pos.col);
+  if(info->protected_cell)
+    printf(" prot");
+  if(info->dwl)
+    printf(" dwl");
+  if(info->dhl)
+    printf(" dhl-%s", info->dhl == 1 ? "top" : info->dhl == 2 ? "bottom" : "?" );
+  printf("\n");
+
+  return 1;
+}
+
+static int want_state_erase = 0;
+static int state_erase(VTermRect rect, int selective, void *user)
+{
+  if(!want_state_erase)
+    return 1;
+
+  printf("erase %d..%d,%d..%d%s\n",
+      rect.start_row, rect.end_row, rect.start_col, rect.end_col,
+      selective ? " selective" : "");
+
+  return 1;
+}
+
+static struct {
+  int bold;
+  int underline;
+  int italic;
+  int blink;
+  int reverse;
+  int strike;
+  int font;
+  VTermColor foreground;
+  VTermColor background;
+} state_pen;
+static int state_setpenattr(VTermAttr attr, VTermValue *val, void *user)
+{
+  switch(attr) {
+  case VTERM_ATTR_BOLD:
+    state_pen.bold = val->boolean;
+    break;
+  case VTERM_ATTR_UNDERLINE:
+    state_pen.underline = val->number;
+    break;
+  case VTERM_ATTR_ITALIC:
+    state_pen.italic = val->boolean;
+    break;
+  case VTERM_ATTR_BLINK:
+    state_pen.blink = val->boolean;
+    break;
+  case VTERM_ATTR_REVERSE:
+    state_pen.reverse = val->boolean;
+    break;
+  case VTERM_ATTR_STRIKE:
+    state_pen.strike = val->boolean;
+    break;
+  case VTERM_ATTR_FONT:
+    state_pen.font = val->number;
+    break;
+  case VTERM_ATTR_FOREGROUND:
+    state_pen.foreground = val->color;
+    break;
+  case VTERM_ATTR_BACKGROUND:
+    state_pen.background = val->color;
+    break;
+  }
+
+  return 1;
+}
+
+static int state_setlineinfo(int row, const VTermLineInfo *newinfo, const VTermLineInfo *oldinfo, void *user)
+{
+  return 1;
+}
+
+VTermStateCallbacks state_cbs = {
+  state_putglyph, /* putglyph */
+  movecursor, /* movecursor */
+  scrollrect, /* scrollrect */
+  moverect, /* moverect */
+  state_erase, /* erase */
+  NULL, /* initpen */
+  state_setpenattr, /* setpenattr */
+  settermprop, /* settermprop */
+  NULL, /* bell */
+  NULL, /* resize */
+  state_setlineinfo, /* setlineinfo */
+};
+
+static int want_screen_damage = 0;
+static int want_screen_damage_cells = 0;
+static int screen_damage(VTermRect rect, void *user)
+{
+  if(!want_screen_damage)
+    return 1;
+
+  printf("damage %d..%d,%d..%d",
+      rect.start_row, rect.end_row, rect.start_col, rect.end_col);
+
+  if(want_screen_damage_cells) {
+    bool equals = false;
+    int row;
+    int col;
+
+    for(row = rect.start_row; row < rect.end_row; row++) {
+      int eol = rect.end_col;
+      while(eol > rect.start_col) {
+        VTermScreenCell cell;
+	VTermPos pos;
+	pos.row = row;
+	pos.col = eol-1;
+        vterm_screen_get_cell(screen, pos, &cell);
+        if(cell.chars[0])
+          break;
+
+        eol--;
+      }
+
+      if(eol == rect.start_col)
+        break;
+
+      if(!equals)
+        printf(" ="), equals = true;
+
+      printf(" %d<", row);
+      for(col = rect.start_col; col < eol; col++) {
+        VTermScreenCell cell;
+	VTermPos pos;
+	pos.row = row;
+	pos.col = col;
+        vterm_screen_get_cell(screen, pos, &cell);
+        printf(col == rect.start_col ? "%02X" : " %02X", cell.chars[0]);
+      }
+      printf(">");
+    }
+  }
+
+  printf("\n");
+
+  return 1;
+}
+
+static int want_screen_scrollback = 0;
+static int screen_sb_pushline(int cols, const VTermScreenCell *cells, void *user)
+{
+  int eol;
+  int c;
+
+  if(!want_screen_scrollback)
+    return 1;
+
+  eol = cols;
+  while(eol && !cells[eol-1].chars[0])
+    eol--;
+
+  printf("sb_pushline %d =", cols);
+  for(c = 0; c < eol; c++)
+    printf(" %02X", cells[c].chars[0]);
+  printf("\n");
+
+  return 1;
+}
+
+static int screen_sb_popline(int cols, VTermScreenCell *cells, void *user)
+{
+  int col;
+
+  if(!want_screen_scrollback)
+    return 0;
+
+  /* All lines of scrollback contain "ABCDE" */
+  for(col = 0; col < cols; col++) {
+    if(col < 5)
+      cells[col].chars[0] = 'A' + col;
+    else
+      cells[col].chars[0] = 0;
+
+    cells[col].width = 1;
+  }
+
+  printf("sb_popline %d\n", cols);
+  return 1;
+}
+
+VTermScreenCallbacks screen_cbs = {
+  screen_damage, /* damage */
+  moverect, /* moverect */
+  movecursor, /* movecursor */
+  settermprop, /* settermprop */
+  NULL, /* bell */
+  NULL, /* resize */
+  screen_sb_pushline, /* sb_pushline */
+  screen_sb_popline /* sb_popline */
+};
+
+int main(int argc, char **argv)
+{
+  char line[1024] = {0};
+  int flag;
+
+  int err;
+
+  setvbuf(stdout, NULL, _IONBF, 0);
+
+  while(fgets(line, sizeof line, stdin)) {
+    char *nl;
+    size_t outlen;
+    err = 0;
+
+    if((nl = strchr(line, '\n')))
+      *nl = '\0';
+
+    if(streq(line, "INIT")) {
+      if(!vt)
+        vt = vterm_new(25, 80);
+    }
+
+    else if(streq(line, "WANTPARSER")) {
+      vterm_parser_set_callbacks(vt, &parser_cbs, NULL);
+    }
+
+    else if(strstartswith(line, "WANTSTATE") && (line[9] == '\0' || line[9] == ' ')) {
+      int i = 9;
+      int sense = 1;
+      if(!state) {
+        state = vterm_obtain_state(vt);
+        vterm_state_set_callbacks(state, &state_cbs, NULL);
+        vterm_state_set_bold_highbright(state, 1);
+        vterm_state_reset(state, 1);
+      }
+
+      while(line[i] == ' ')
+        i++;
+      for( ; line[i]; i++)
+        switch(line[i]) {
+        case '+':
+          sense = 1;
+          break;
+        case '-':
+          sense = 0;
+          break;
+        case 'g':
+          want_state_putglyph = sense;
+          break;
+        case 's':
+          want_scrollrect = sense;
+          break;
+        case 'm':
+          want_moverect = sense;
+          break;
+        case 'e':
+          want_state_erase = sense;
+          break;
+        case 'p':
+          want_settermprop = sense;
+          break;
+        case 'f':
+          vterm_state_set_unrecognised_fallbacks(state, sense ? &parser_cbs : NULL, NULL);
+          break;
+        default:
+          fprintf(stderr, "Unrecognised WANTSTATE flag '%c'\n", line[i]);
+        }
+    }
+
+    else if(strstartswith(line, "WANTSCREEN") && (line[10] == '\0' || line[10] == ' ')) {
+      int i = 10;
+      int sense = 1;
+      if(!screen)
+        screen = vterm_obtain_screen(vt);
+      vterm_screen_enable_altscreen(screen, 1);
+      vterm_screen_set_callbacks(screen, &screen_cbs, NULL);
+
+      while(line[i] == ' ')
+        i++;
+      for( ; line[i]; i++)
+        switch(line[i]) {
+        case '-':
+          sense = 0;
+          break;
+        case 'd':
+          want_screen_damage = sense;
+          break;
+        case 'D':
+          want_screen_damage = sense;
+          want_screen_damage_cells = sense;
+          break;
+        case 'm':
+          want_moverect = sense;
+          break;
+        case 'c':
+          want_movecursor = sense;
+          break;
+        case 'p':
+          want_settermprop = 1;
+          break;
+        case 'b':
+          want_screen_scrollback = sense;
+          break;
+        default:
+          fprintf(stderr, "Unrecognised WANTSCREEN flag '%c'\n", line[i]);
+        }
+    }
+
+    else if(sscanf(line, "UTF8 %d", &flag)) {
+      vterm_set_utf8(vt, flag);
+    }
+
+    else if(streq(line, "RESET")) {
+      if(state) {
+        vterm_state_reset(state, 1);
+        vterm_state_get_cursorpos(state, &state_pos);
+      }
+      if(screen) {
+        vterm_screen_reset(screen, 1);
+      }
+    }
+
+    else if(strstartswith(line, "RESIZE ")) {
+      int rows, cols;
+      char *linep = line + 7;
+      while(linep[0] == ' ')
+        linep++;
+      sscanf(linep, "%d, %d", &rows, &cols);
+      vterm_set_size(vt, rows, cols);
+    }
+
+    else if(strstartswith(line, "PUSH ")) {
+      char *bytes = line + 5;
+      size_t len = inplace_hex2bytes(bytes);
+      size_t written = vterm_input_write(vt, bytes, len);
+      if(written < len)
+        fprintf(stderr, "! short write\n");
+    }
+
+    else if(streq(line, "WANTENCODING")) {
+      /* This isn't really external API but it's hard to get this out any
+       * other way
+       */
+      encoding.enc = vterm_lookup_encoding(ENC_UTF8, 'u');
+      if(encoding.enc->init)
+        (*encoding.enc->init)(encoding.enc, encoding.data);
+    }
+
+    else if(strstartswith(line, "ENCIN ")) {
+      char *bytes = line + 6;
+      size_t len = inplace_hex2bytes(bytes);
+
+      uint32_t cp[1024];
+      int cpi = 0;
+      size_t pos = 0;
+
+      (*encoding.enc->decode)(encoding.enc, encoding.data,
+          cp, &cpi, len, bytes, &pos, len);
+
+      if(cpi > 0) {
+	int i;
+        printf("encout ");
+        for(i = 0; i < cpi; i++) {
+          printf(i ? ",%x" : "%x", cp[i]);
+        }
+        printf("\n");
+      }
+    }
+
+    else if(strstartswith(line, "INCHAR ")) {
+      char *linep = line + 7;
+      unsigned int c = 0;
+      VTermModifier mod;
+      while(linep[0] == ' ')
+        linep++;
+      mod = strpe_modifiers(&linep);
+      sscanf(linep, " %x", &c);
+
+      vterm_keyboard_unichar(vt, c, mod);
+    }
+
+    else if(strstartswith(line, "INKEY ")) {
+      VTermModifier mod;
+      VTermKey key;
+      char *linep = line + 6;
+      while(linep[0] == ' ')
+        linep++;
+      mod = strpe_modifiers(&linep);
+      while(linep[0] == ' ')
+        linep++;
+      key = strp_key(linep);
+
+      vterm_keyboard_key(vt, key, mod);
+    }
+
+    else if(strstartswith(line, "PASTE ")) {
+      char *linep = line + 6;
+      if(streq(linep, "START"))
+        vterm_keyboard_start_paste(vt);
+      else if(streq(linep, "END"))
+        vterm_keyboard_end_paste(vt);
+      else
+        goto abort_line;
+    }
+
+    else if(strstartswith(line, "MOUSEMOVE ")) {
+      char *linep = line + 10;
+      int row, col, len;
+      VTermModifier mod;
+      while(linep[0] == ' ')
+        linep++;
+      sscanf(linep, "%d,%d%n", &row, &col, &len);
+      linep += len;
+      while(linep[0] == ' ')
+        linep++;
+      mod = strpe_modifiers(&linep);
+      vterm_mouse_move(vt, row, col, mod);
+    }
+
+    else if(strstartswith(line, "MOUSEBTN ")) {
+      char *linep = line + 9;
+      char press;
+      int button, len;
+      VTermModifier mod;
+      while(linep[0] == ' ')
+        linep++;
+      sscanf(linep, "%c %d%n", &press, &button, &len);
+      linep += len;
+      while(linep[0] == ' ')
+        linep++;
+      mod = strpe_modifiers(&linep);
+      vterm_mouse_button(vt, button, (press == 'd' || press == 'D'), mod);
+    }
+
+    else if(strstartswith(line, "DAMAGEMERGE ")) {
+      char *linep = line + 12;
+      while(linep[0] == ' ')
+        linep++;
+      if(streq(linep, "CELL"))
+        vterm_screen_set_damage_merge(screen, VTERM_DAMAGE_CELL);
+      else if(streq(linep, "ROW"))
+        vterm_screen_set_damage_merge(screen, VTERM_DAMAGE_ROW);
+      else if(streq(linep, "SCREEN"))
+        vterm_screen_set_damage_merge(screen, VTERM_DAMAGE_SCREEN);
+      else if(streq(linep, "SCROLL"))
+        vterm_screen_set_damage_merge(screen, VTERM_DAMAGE_SCROLL);
+    }
+
+    else if(strstartswith(line, "DAMAGEFLUSH")) {
+      vterm_screen_flush_damage(screen);
+    }
+
+    else if(line[0] == '?') {
+      if(streq(line, "?cursor")) {
+        VTermPos pos;
+        vterm_state_get_cursorpos(state, &pos);
+        if(pos.row != state_pos.row)
+          printf("! row mismatch: state=%d,%d event=%d,%d\n",
+              pos.row, pos.col, state_pos.row, state_pos.col);
+        else if(pos.col != state_pos.col)
+          printf("! col mismatch: state=%d,%d event=%d,%d\n",
+              pos.row, pos.col, state_pos.row, state_pos.col);
+        else
+          printf("%d,%d\n", state_pos.row, state_pos.col);
+      }
+      else if(strstartswith(line, "?pen ")) {
+        VTermValue val;
+        char *linep = line + 5;
+        while(linep[0] == ' ')
+          linep++;
+
+#define BOOLSTR(v) ((v) ? "on" : "off")
+
+        if(streq(linep, "bold")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_BOLD, &val);
+          if(val.boolean != state_pen.bold)
+            printf("! pen bold mismatch; state=%s, event=%s\n",
+                BOOLSTR(val.boolean), BOOLSTR(state_pen.bold));
+          else
+            printf("%s\n", BOOLSTR(state_pen.bold));
+        }
+        else if(streq(linep, "underline")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_UNDERLINE, &val);
+          if(val.boolean != state_pen.underline)
+            printf("! pen underline mismatch; state=%d, event=%d\n",
+                val.boolean, state_pen.underline);
+          else
+            printf("%d\n", state_pen.underline);
+        }
+        else if(streq(linep, "italic")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_ITALIC, &val);
+          if(val.boolean != state_pen.italic)
+            printf("! pen italic mismatch; state=%s, event=%s\n",
+                BOOLSTR(val.boolean), BOOLSTR(state_pen.italic));
+          else
+            printf("%s\n", BOOLSTR(state_pen.italic));
+        }
+        else if(streq(linep, "blink")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_BLINK, &val);
+          if(val.boolean != state_pen.blink)
+            printf("! pen blink mismatch; state=%s, event=%s\n",
+                BOOLSTR(val.boolean), BOOLSTR(state_pen.blink));
+          else
+            printf("%s\n", BOOLSTR(state_pen.blink));
+        }
+        else if(streq(linep, "reverse")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_REVERSE, &val);
+          if(val.boolean != state_pen.reverse)
+            printf("! pen reverse mismatch; state=%s, event=%s\n",
+                BOOLSTR(val.boolean), BOOLSTR(state_pen.reverse));
+          else
+            printf("%s\n", BOOLSTR(state_pen.reverse));
+        }
+        else if(streq(linep, "font")) {
+          vterm_state_get_penattr(state, VTERM_ATTR_FONT, &val);
+          if(val.boolean != state_pen.font)
+            printf("! pen font mismatch; state=%d, event=%d\n",
+                val.boolean, state_pen.font);
+          else
+            printf("%d\n", state_pen.font);
+        }
+        else if(streq(linep, "foreground")) {
+          printf("rgb(%d,%d,%d)\n", state_pen.foreground.red, state_pen.foreground.green, state_pen.foreground.blue);
+        }
+        else if(streq(linep, "background")) {
+          printf("rgb(%d,%d,%d)\n", state_pen.background.red, state_pen.background.green, state_pen.background.blue);
+        }
+        else
+          printf("?\n");
+      }
+      else if(strstartswith(line, "?screen_chars ")) {
+        char *linep = line + 13;
+        VTermRect rect;
+        size_t len;
+        while(linep[0] == ' ')
+          linep++;
+        if(sscanf(linep, "%d,%d,%d,%d", &rect.start_row, &rect.start_col, &rect.end_row, &rect.end_col) < 4) {
+          printf("! screen_chars unrecognised input\n");
+          goto abort_line;
+        }
+        len = vterm_screen_get_chars(screen, NULL, 0, rect);
+        if(len == (size_t)-1)
+          printf("! screen_chars error\n");
+        else if(len == 0)
+          printf("\n");
+        else {
+          uint32_t *chars = malloc(sizeof(uint32_t) * len);
+          size_t i;
+          vterm_screen_get_chars(screen, chars, len, rect);
+          for(i = 0; i < len; i++) {
+            printf("0x%02x%s", chars[i], i < len-1 ? "," : "\n");
+          }
+          free(chars);
+        }
+      }
+      else if(strstartswith(line, "?screen_text ")) {
+        char *linep = line + 12;
+        VTermRect rect;
+        size_t len;
+        while(linep[0] == ' ')
+          linep++;
+        if(sscanf(linep, "%d,%d,%d,%d", &rect.start_row, &rect.start_col, &rect.end_row, &rect.end_col) < 4) {
+          printf("! screen_text unrecognised input\n");
+          goto abort_line;
+        }
+        len = vterm_screen_get_text(screen, NULL, 0, rect);
+        if(len == (size_t)-1)
+          printf("! screen_text error\n");
+        else if(len == 0)
+          printf("\n");
+        else {
+          /* Put an overwrite guard at both ends of the buffer */
+          unsigned char *buffer = malloc(len + 4);
+          unsigned char *text = buffer + 2;
+          text[-2] = 0x55; text[-1] = 0xAA;
+          text[len] = 0x55; text[len+1] = 0xAA;
+
+          vterm_screen_get_text(screen, (char *)text, len, rect);
+
+          if(text[-2] != 0x55 || text[-1] != 0xAA)
+            printf("! screen_get_text buffer overrun left [%02x,%02x]\n", text[-2], text[-1]);
+          else if(text[len] != 0x55 || text[len+1] != 0xAA)
+            printf("! screen_get_text buffer overrun right [%02x,%02x]\n", text[len], text[len+1]);
+          else
+	  {
+	    size_t i;
+            for(i = 0; i < len; i++) {
+              printf("0x%02x%s", text[i], i < len-1 ? "," : "\n");
+            }
+	  }
+
+          free(buffer);
+        }
+      }
+      else if(strstartswith(line, "?screen_cell ")) {
+        char *linep = line + 12;
+	int i;
+        VTermPos pos;
+        VTermScreenCell cell;
+        while(linep[0] == ' ')
+          linep++;
+        if(sscanf(linep, "%d,%d\n", &pos.row, &pos.col) < 2) {
+          printf("! screen_cell unrecognised input\n");
+          goto abort_line;
+        }
+        if(!vterm_screen_get_cell(screen, pos, &cell))
+          goto abort_line;
+        printf("{");
+        for(i = 0; i < VTERM_MAX_CHARS_PER_CELL && cell.chars[i]; i++) {
+          printf("%s0x%x", i ? "," : "", cell.chars[i]);
+        }
+        printf("} width=%d attrs={", cell.width);
+        if(cell.attrs.bold)      printf("B");
+        if(cell.attrs.underline) printf("U%d", cell.attrs.underline);
+        if(cell.attrs.italic)    printf("I");
+        if(cell.attrs.blink)     printf("K");
+        if(cell.attrs.reverse)   printf("R");
+        if(cell.attrs.font)      printf("F%d", cell.attrs.font);
+        printf("} ");
+        if(cell.attrs.dwl)       printf("dwl ");
+        if(cell.attrs.dhl)       printf("dhl-%s ", cell.attrs.dhl == 2 ? "bottom" : "top");
+        printf("fg=rgb(%d,%d,%d) ",  cell.fg.red, cell.fg.green, cell.fg.blue);
+        printf("bg=rgb(%d,%d,%d)\n", cell.bg.red, cell.bg.green, cell.bg.blue);
+      }
+      else if(strstartswith(line, "?screen_eol ")) {
+        VTermPos pos;
+        char *linep = line + 12;
+        while(linep[0] == ' ')
+          linep++;
+        if(sscanf(linep, "%d,%d\n", &pos.row, &pos.col) < 2) {
+          printf("! screen_eol unrecognised input\n");
+          goto abort_line;
+        }
+        printf("%d\n", vterm_screen_is_eol(screen, pos));
+      }
+      else if(strstartswith(line, "?screen_attrs_extent ")) {
+        VTermPos pos;
+        VTermRect rect;
+        char *linep = line + 21;
+        while(linep[0] == ' ')
+          linep++;
+        if(sscanf(linep, "%d,%d\n", &pos.row, &pos.col) < 2) {
+          printf("! screen_attrs_extent unrecognised input\n");
+          goto abort_line;
+        }
+	rect.start_col = 0;
+	rect.end_col   = -1;
+        if(!vterm_screen_get_attrs_extent(screen, &rect, pos, ~0)) {
+          printf("! screen_attrs_extent failed\n");
+          goto abort_line;
+        }
+        printf("%d,%d-%d,%d\n", rect.start_row, rect.start_col, rect.end_row, rect.end_col);
+      }
+      else
+        printf("?\n");
+
+      memset(line, 0, sizeof line);
+      continue;
+    }
+
+    else
+      abort_line: err = 1;
+
+    outlen = vterm_output_get_buffer_current(vt);
+    if(outlen > 0) {
+      int i;
+      char outbuff[1024];
+      vterm_output_read(vt, outbuff, outlen);
+
+      printf("output ");
+      for(i = 0; i < outlen; i++)
+        printf("%x%s", (unsigned char)outbuff[i], i < outlen-1 ? "," : "\n");
+    }
+
+    printf(err ? "?\n" : "DONE\n");
+  }
+
+  vterm_free(vt);
+
+  return 0;
+}