diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index 94561e5..517ed2b 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -1246,6 +1246,7 @@
 
 CCCTERM = $(CC) -c $(CFLAGS) -Ilibvterm/include -DINLINE="" \
 	  -DVSNPRINTF=vim_vsnprintf \
+	  -DSNPRINTF=vim_snprintf \
 	  -DIS_COMBINING_FUNCTION=utf_iscomposing_uint \
 	  -DWCWIDTH_FUNCTION=utf_uint2cells \
 	  -DGET_SPECIAL_PTY_TYPE_FUNCTION=get_special_pty_type
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index c93517b..8240f50 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -1812,6 +1812,7 @@
 
 CCCTERM = $(CC) $(CFLAGS) -Ilibvterm/include -DINLINE="" \
 	-DVSNPRINTF=vim_vsnprintf \
+	-DSNPRINTF=vim_snprintf \
 	-DIS_COMBINING_FUNCTION=utf_iscomposing_uint \
 	-DWCWIDTH_FUNCTION=utf_uint2cells \
 	-DGET_SPECIAL_PTY_TYPE_FUNCTION=get_special_pty_type \
diff --git a/src/Makefile b/src/Makefile
index 7c2934a..ad8f241 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3537,6 +3537,7 @@
 # prefix vterm_ to avoid name clashes.
 CCCTERM = $(CCC_NF) $(VTERM_CFLAGS) $(ALL_CFLAGS) -DINLINE="" \
 	  -DVSNPRINTF=vim_vsnprintf \
+	  -DSNPRINTF=vim_snprintf \
 	  -DIS_COMBINING_FUNCTION=utf_iscomposing_uint \
 	  -DWCWIDTH_FUNCTION=utf_uint2cells
 
diff --git a/src/libvterm/src/keyboard.c b/src/libvterm/src/keyboard.c
index 95b962e..9c2a53d 100644
--- a/src/libvterm/src/keyboard.c
+++ b/src/libvterm/src/keyboard.c
@@ -102,10 +102,10 @@
 
 static keycodes_s keycodes_fn[] = {
   { KEYCODE_NONE,       0, 0 },   // F0 - shouldn't happen
-  { KEYCODE_CSI_CURSOR, 'P', 0 }, // F1
-  { KEYCODE_CSI_CURSOR, 'Q', 0 }, // F2
-  { KEYCODE_CSI_CURSOR, 'R', 0 }, // F3
-  { KEYCODE_CSI_CURSOR, 'S', 0 }, // F4
+  { KEYCODE_SS3,	'P', 0 }, // F1
+  { KEYCODE_SS3,	'Q', 0 }, // F2
+  { KEYCODE_SS3,	'R', 0 }, // F3
+  { KEYCODE_SS3,	'S', 0 }, // F4
   { KEYCODE_CSINUM,     '~', 15 }, // F5
   { KEYCODE_CSINUM,     '~', 17 }, // F6
   { KEYCODE_CSINUM,     '~', 18 }, // F7
diff --git a/src/libvterm/src/vterm.c b/src/libvterm/src/vterm.c
index 1d6a977..56c4e28 100644
--- a/src/libvterm/src/vterm.c
+++ b/src/libvterm/src/vterm.c
@@ -74,6 +74,9 @@
     return NULL;
   }
 
+  vt->tmpbuffer_len = 64;
+  vt->tmpbuffer = vterm_allocator_malloc(vt, vt->tmpbuffer_len);
+
   return vt;
 }
 
@@ -135,67 +138,46 @@
 INTERNAL void vterm_push_output_bytes(VTerm *vt, const char *bytes, size_t len)
 {
   if(len > vt->outbuffer_len - vt->outbuffer_cur) {
-    DEBUG_LOG("vterm_push_output(): buffer overflow; truncating output\n");
-    len = vt->outbuffer_len - vt->outbuffer_cur;
+    DEBUG_LOG("vterm_push_output_bytes(): buffer overflow; dropping output\n");
+    return;
   }
 
   memcpy(vt->outbuffer + vt->outbuffer_cur, bytes, len);
   vt->outbuffer_cur += len;
 }
 
-static int outbuffer_is_full(VTerm *vt)
-{
-  return vt->outbuffer_cur >= vt->outbuffer_len - 1;
-}
-
 #if (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 500) \
 	|| defined(_ISOC99_SOURCE) || defined(_BSD_SOURCE)
 # undef VSNPRINTF
 # define VSNPRINTF vsnprintf
+# undef SNPRINTF
+# define SNPRINTF snprintf
 #else
 # ifdef VSNPRINTF
 // Use a provided vsnprintf() function.
 int VSNPRINTF(char *str, size_t str_m, const char *fmt, va_list ap);
 # endif
+# ifdef SNPRINTF
+// Use a provided snprintf() function.
+int SNPRINTF(char *str, size_t str_m, const char *fmt, ...);
+# endif
 #endif
 
 
 INTERNAL void vterm_push_output_vsprintf(VTerm *vt, const char *format, va_list args)
 {
-  int written;
+  size_t len;
 #ifndef VSNPRINTF
   // When vsnprintf() is not available (C90) fall back to vsprintf().
   char buffer[1024]; // 1Kbyte is enough for everybody, right?
 #endif
 
-  if(outbuffer_is_full(vt)) {
-    DEBUG_LOG("vterm_push_output(): buffer overflow; truncating output\n");
-    return;
-  }
-
 #ifdef VSNPRINTF
-  written = VSNPRINTF(vt->outbuffer + vt->outbuffer_cur,
-      vt->outbuffer_len - vt->outbuffer_cur,
-      format, args);
-
-  if(written == (int)(vt->outbuffer_len - vt->outbuffer_cur)) {
-    // output was truncated
-    vt->outbuffer_cur = vt->outbuffer_len - 1;
-  }
-  else
-    vt->outbuffer_cur += written;
+  len = VSNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len, format, args);
+  vterm_push_output_bytes(vt, vt->tmpbuffer, len);
 #else
-  written = vsprintf(buffer, format, args);
-
-  if(written >= (int)(vt->outbuffer_len - vt->outbuffer_cur - 1)) {
-    // output was truncated
-    written = vt->outbuffer_len - vt->outbuffer_cur - 1;
-  }
-  if (written > 0)
-  {
-    strncpy(vt->outbuffer + vt->outbuffer_cur, buffer, written + 1);
-    vt->outbuffer_cur += written;
-  }
+  len = vsprintf(buffer, format, args);
+  vterm_push_output_bytes(vt, buffer, len);
 #endif
 }
 
@@ -209,40 +191,52 @@
 
 INTERNAL void vterm_push_output_sprintf_ctrl(VTerm *vt, unsigned char ctrl, const char *fmt, ...)
 {
-  size_t orig_cur = vt->outbuffer_cur;
+  size_t cur;
   va_list args;
 
   if(ctrl >= 0x80 && !vt->mode.ctrl8bit)
-    vterm_push_output_sprintf(vt, ESC_S "%c", ctrl - 0x40);
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        ESC_S "%c", ctrl - 0x40);
   else
-    vterm_push_output_sprintf(vt, "%c", ctrl);
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        "%c", ctrl);
+  if(cur >= vt->tmpbuffer_len)
+    return;
+  vterm_push_output_bytes(vt, vt->tmpbuffer, cur);
 
   va_start(args, fmt);
   vterm_push_output_vsprintf(vt, fmt, args);
   va_end(args);
-
-  if(outbuffer_is_full(vt))
-    vt->outbuffer_cur = orig_cur;
 }
 
 INTERNAL void vterm_push_output_sprintf_dcs(VTerm *vt, const char *fmt, ...)
 {
-  size_t orig_cur = vt->outbuffer_cur;
+  size_t cur;
   va_list args;
 
   if(!vt->mode.ctrl8bit)
-    vterm_push_output_sprintf(vt, ESC_S "%c", C1_DCS - 0x40);
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        ESC_S "%c", C1_DCS - 0x40);
   else
-    vterm_push_output_sprintf(vt, "%c", C1_DCS);
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        "%c", C1_DCS);
+  if(cur >= vt->tmpbuffer_len)
+    return;
+  vterm_push_output_bytes(vt, vt->tmpbuffer, cur);
 
   va_start(args, fmt);
   vterm_push_output_vsprintf(vt, fmt, args);
   va_end(args);
 
-  vterm_push_output_sprintf_ctrl(vt, C1_ST, "");
-
-  if(outbuffer_is_full(vt))
-    vt->outbuffer_cur = orig_cur;
+  if(!vt->mode.ctrl8bit)
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        ESC_S "%c", C1_ST - 0x40);
+  else
+    cur = SNPRINTF(vt->tmpbuffer, vt->tmpbuffer_len,
+        "%c", C1_ST);
+  if(cur >= vt->tmpbuffer_len)
+    return;
+  vterm_push_output_bytes(vt, vt->tmpbuffer, cur);
 }
 
 size_t vterm_output_get_buffer_size(const VTerm *vt)
diff --git a/src/libvterm/src/vterm_internal.h b/src/libvterm/src/vterm_internal.h
index e2b8b15..6aa1e9c 100644
--- a/src/libvterm/src/vterm_internal.h
+++ b/src/libvterm/src/vterm_internal.h
@@ -211,6 +211,9 @@
   size_t outbuffer_len;
   size_t outbuffer_cur;
 
+  char  *tmpbuffer;
+  size_t tmpbuffer_len;
+
   VTermState *state;
   VTermScreen *screen;
 
diff --git a/src/libvterm/t/25state_input.test b/src/libvterm/t/25state_input.test
index a5119fb..4eb4c6a 100644
--- a/src/libvterm/t/25state_input.test
+++ b/src/libvterm/t/25state_input.test
@@ -111,6 +111,18 @@
 INKEY 0 Enter
   output "\x0d\x0a"
 
+!Unmodified F1 is SS3 P
+INKEY 0 F1
+  output "\eOP"
+
+!Modified F1 is CSI P
+INKEY S F1
+  output "\e[1;2P"
+INKEY A F1
+  output "\e[1;3P"
+INKEY C F1
+  output "\e[1;5P"
+
 !Keypad in DECKPNM
 INKEY 0 KP0
   output "0"
diff --git a/src/libvterm/t/26state_query.test b/src/libvterm/t/26state_query.test
index 3ace2d5..c6be90b 100644
--- a/src/libvterm/t/26state_query.test
+++ b/src/libvterm/t/26state_query.test
@@ -59,4 +59,4 @@
 
 !Truncation on attempted buffer overflow
 PUSH "\e[6n" x 30
-  output "\e[10;10R" x 24
+  output "\e[10;10R" x 25
diff --git a/src/libvterm/t/harness.c b/src/libvterm/t/harness.c
index 75869d7..e28bb7d 100644
--- a/src/libvterm/t/harness.c
+++ b/src/libvterm/t/harness.c
@@ -47,6 +47,7 @@
     { "Tab",   VTERM_KEY_TAB },
     { "Enter", VTERM_KEY_ENTER },
     { "KP0",   VTERM_KEY_KP_0 },
+    { "F1",    VTERM_KEY_FUNCTION(1) },
     { NULL,    VTERM_KEY_NONE },
   };
   int i;
diff --git a/src/version.c b/src/version.c
index f047bde..ee9a393 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    783,
+/**/
     782,
 /**/
     781,