Merge #6142 from justinmk/term-modifiable

terminal: 'modifiable'; 'scrollback'; follow output only if cursor is on last line

11 files changed, 190 insertions, 176 deletions

diff --git a/src/.valgrind.supp b/src/.valgrind.supp
index 8b630fcaaf..cce22bd632 100644
--- a/src/.valgrind.supp
+++ b/src/.valgrind.supp
@@ -10,7 +10,7 @@
   Memcheck:Leak
   fun:malloc
   fun:uv_spawn
-  fun:pipe_process_spawn
+  fun:libuv_process_spawn
   fun:process_spawn
   fun:job_start
 }
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 9c5c7859ff..ccdab16ca1 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -36,7 +36,7 @@ typedef struct {
 // for Map(K, V)
 #include "nvim/map.h"
 
-#define MODIFIABLE(buf) (!buf->terminal && buf->b_p_ma)
+#define MODIFIABLE(buf) (buf->b_p_ma)
 
 /*
  * Flags for w_valid.
@@ -91,32 +91,22 @@ typedef struct frame_S frame_T;
 
 // for struct memline (it needs memfile_T)
 #include "nvim/memline_defs.h"
-
 // for struct memfile, bhdr_T, blocknr_T... (it needs buf_T)
 #include "nvim/memfile_defs.h"
 
-/*
- * This is here because regexp_defs.h needs win_T and buf_T. regprog_T is
- * used below.
- */
+// for regprog_T. Needs win_T and buf_T.
 #include "nvim/regexp_defs.h"
-
-// for  synstate_T (needs reg_extmatch_T, win_T and buf_T)
+// for synstate_T (needs reg_extmatch_T, win_T, buf_T)
 #include "nvim/syntax_defs.h"
-
 // for signlist_T
 #include "nvim/sign_defs.h"
-
 // for bufhl_*_T
 #include "nvim/bufhl_defs.h"
 
 typedef Map(linenr_T, bufhl_vec_T) bufhl_info_T;
 
-// for FileID
-#include "nvim/os/fs_defs.h"
-
-// for Terminal
-#include "nvim/terminal.h"
+#include "nvim/os/fs_defs.h"    // for FileID
+#include "nvim/terminal.h"      // for Terminal
 
 /*
  * The taggy struct is used to store the information about a :tag command.
@@ -661,6 +651,7 @@ struct file_buffer {
   char_u *b_p_qe;               ///< 'quoteescape'
   int b_p_ro;                   ///< 'readonly'
   long b_p_sw;                  ///< 'shiftwidth'
+  long b_p_scbk;                ///< 'scrollback'
   int b_p_si;                   ///< 'smartindent'
   long b_p_sts;                 ///< 'softtabstop'
   long b_p_sts_nopaste;         ///< b_p_sts saved for paste mode
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 57c2368523..49644d70ef 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -5845,8 +5845,8 @@ bool garbage_collect(bool testing)
     garbage_collect_at_exit = false;
   }
 
-  // We advance by two because we add one for items referenced through
-  // previous_funccal.
+  // We advance by two (COPYID_INC) because we add one for items referenced
+  // through previous_funccal.
   const int copyID = get_copyID();
 
   // 1. Go through all accessible variables and mark all lists and dicts
diff --git a/src/nvim/msgpack_rpc/channel.c b/src/nvim/msgpack_rpc/channel.c
index 98636263b9..71db7506ad 100644
--- a/src/nvim/msgpack_rpc/channel.c
+++ b/src/nvim/msgpack_rpc/channel.c
@@ -321,14 +321,15 @@ static void parse_msgpack(Stream *stream, RBuffer *rbuf, size_t c, void *data,
 
   if (eof) {
     close_channel(channel);
-    call_set_error(channel, "Channel was closed by the client");
+    char buf[256];
+    snprintf(buf, sizeof(buf), "channel %" PRIu64 " was closed by the client",
+             channel->id);
+    call_set_error(channel, buf);
     goto end;
   }
 
   size_t count = rbuffer_size(rbuf);
-  DLOG("Feeding the msgpack parser with %u bytes of data from Stream(%p)",
-       count,
-       stream);
+  DLOG("parsing %u bytes of msgpack data from Stream(%p)", count, stream);
 
   // Feed the unpacker with data
   msgpack_unpacker_reserve_buffer(channel->unpacker, count);
@@ -350,11 +351,9 @@ static void parse_msgpack(Stream *stream, RBuffer *rbuf, size_t c, void *data,
         complete_call(&unpacked.data, channel);
       } else {
         char buf[256];
-        snprintf(buf,
-                 sizeof(buf),
-                 "Channel %" PRIu64 " returned a response that doesn't have "
-                 "a matching request id. Ensure the client is properly "
-                 "synchronized",
+        snprintf(buf, sizeof(buf),
+                 "channel %" PRIu64 " sent a response without a matching "
+                 "request id. Ensure the client is properly synchronized",
                  channel->id);
         call_set_error(channel, buf);
       }
@@ -406,7 +405,7 @@ static void handle_request(Channel *channel, msgpack_object *request)
                                          &out_buffer))) {
       char buf[256];
       snprintf(buf, sizeof(buf),
-               "Channel %" PRIu64 " sent an invalid message, closed.",
+               "channel %" PRIu64 " sent an invalid message, closed.",
                channel->id);
       call_set_error(channel, buf);
     }
@@ -716,7 +715,7 @@ static void complete_call(msgpack_object *obj, Channel *channel)
 
 static void call_set_error(Channel *channel, char *msg)
 {
-  ELOG("msgpack-rpc: %s", msg);
+  ELOG("RPC: %s", msg);
   for (size_t i = 0; i < kv_size(channel->call_stack); i++) {
     ChannelCallFrame *frame = kv_A(channel->call_stack, i);
     frame->returned = true;
@@ -789,10 +788,10 @@ static void decref(Channel *channel)
 }
 
 #if MIN_LOG_LEVEL <= DEBUG_LOG_LEVEL
-#define REQ "[request]      "
-#define RES "[response]     "
-#define NOT "[notification] "
-#define ERR "[error]        "
+#define REQ "[request]  "
+#define RES "[response] "
+#define NOT "[notify]   "
+#define ERR "[error]    "
 
 // Cannot define array with negative offsets, so this one is needed to be added
 // to MSGPACK_UNPACK_\* values.
@@ -810,7 +809,7 @@ static void log_server_msg(uint64_t channel_id,
 {
   msgpack_unpacked unpacked;
   msgpack_unpacked_init(&unpacked);
-  DLOGN("[msgpack-rpc] nvim -> client(%" PRIu64 ") ", channel_id);
+  DLOGN("RPC ->ch %" PRIu64 ": ", channel_id);
   const msgpack_unpack_return result =
       msgpack_unpack_next(&unpacked, packed->data, packed->size, NULL);
   switch (result) {
@@ -847,7 +846,7 @@ static void log_client_msg(uint64_t channel_id,
                            bool is_request,
                            msgpack_object msg)
 {
-  DLOGN("[msgpack-rpc] client(%" PRIu64 ") -> nvim ", channel_id);
+  DLOGN("RPC <-ch %" PRIu64 ": ", channel_id);
   log_lock();
   FILE *f = open_log_file();
   fprintf(f, is_request ? REQ : RES);
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index 86aefda2b2..dbd8e153a8 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -7418,27 +7418,23 @@ static void nv_esc(cmdarg_T *cap)
     restart_edit = 'a';
 }
 
-/*
- * Handle "A", "a", "I", "i" and <Insert> commands.
- */
+/// Handle "A", "a", "I", "i" and <Insert> commands.
 static void nv_edit(cmdarg_T *cap)
 {
-  /* <Insert> is equal to "i" */
-  if (cap->cmdchar == K_INS || cap->cmdchar == K_KINS)
+  // <Insert> is equal to "i"
+  if (cap->cmdchar == K_INS || cap->cmdchar == K_KINS) {
     cap->cmdchar = 'i';
+  }
 
-  /* in Visual mode "A" and "I" are an operator */
-  if (VIsual_active && (cap->cmdchar == 'A' || cap->cmdchar == 'I'))
+  // in Visual mode "A" and "I" are an operator
+  if (VIsual_active && (cap->cmdchar == 'A' || cap->cmdchar == 'I')) {
     v_visop(cap);
-
-  /* in Visual mode and after an operator "a" and "i" are for text objects */
-  else if ((cap->cmdchar == 'a' || cap->cmdchar == 'i')
-           && (cap->oap->op_type != OP_NOP
-               || VIsual_active
-               )) {
+  // in Visual mode and after an operator "a" and "i" are for text objects
+  } else if ((cap->cmdchar == 'a' || cap->cmdchar == 'i')
+             && (cap->oap->op_type != OP_NOP || VIsual_active)) {
     nv_object(cap);
-  } else if (!curbuf->b_p_ma && !p_im) {
-    /* Only give this error when 'insertmode' is off. */
+  } else if (!curbuf->b_p_ma && !p_im && !curbuf->terminal) {
+    // Only give this error when 'insertmode' is off.
     EMSG(_(e_modifiable));
     clearop(cap->oap);
   } else if (!checkclearopq(cap->oap)) {
diff --git a/src/nvim/option.c b/src/nvim/option.c
index e697ab3f51..8990b59f57 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -1,24 +1,18 @@
-/*
- * Code to handle user-settable options. This is all pretty much table-
- * driven. Checklist for adding a new option:
- * - Put it in the options array below (copy an existing entry).
- * - For a global option: Add a variable for it in option_defs.h.
- * - For a buffer or window local option:
- *   - Add a PV_XX entry to the enum below.
- *   - Add a variable to the window or buffer struct in buffer_defs.h.
- *   - For a window option, add some code to copy_winopt().
- *   - For a buffer option, add some code to buf_copy_options().
- *   - For a buffer string option, add code to check_buf_options().
- * - If it's a numeric option, add any necessary bounds checks to do_set().
- * - If it's a list of flags, add some code in do_set(), search for WW_ALL.
- * - When adding an option with expansion (P_EXPAND), but with a different
- *   default for Vi and Vim (no P_VI_DEF), add some code at VIMEXP.
- * - Add documentation!  One line in doc/help.txt, full description in
- *   options.txt, and any other related places.
- * - Add an entry in runtime/optwin.vim.
- * When making changes:
- * - Adjust the help for the option in doc/option.txt.
- */
+// User-settable options. Checklist for adding a new option:
+// - Put it in options.lua
+// - For a global option: Add a variable for it in option_defs.h.
+// - For a buffer or window local option:
+//   - Add a BV_XX or WV_XX entry to option_defs.h
+//   - Add a variable to the window or buffer struct in buffer_defs.h.
+//   - For a window option, add some code to copy_winopt().
+//   - For a buffer option, add some code to buf_copy_options().
+//   - For a buffer string option, add code to check_buf_options().
+// - If it's a numeric option, add any necessary bounds checks to do_set().
+// - If it's a list of flags, add some code in do_set(), search for WW_ALL.
+// - When adding an option with expansion (P_EXPAND), but with a different
+//   default for Vi and Vim (no P_VI_DEF), add some code at VIMEXP.
+// - Add documentation! doc/options.txt, and any other related places.
+// - Add an entry in runtime/optwin.vim.
 
 #define IN_OPTION_C
 #include <assert.h>
@@ -161,6 +155,7 @@ static long p_ts;
 static long p_tw;
 static int p_udf;
 static long p_wm;
+static long p_scbk;
 static char_u   *p_keymap;
 
 /* Saved values for when 'bin' is set. */
@@ -4201,7 +4196,19 @@ set_num_option (
     FOR_ALL_TAB_WINDOWS(tp, wp) {
       check_colorcolumn(wp);
     }
-
+  } else if (pp == &curbuf->b_p_scbk) {
+    // 'scrollback'
+    if (!curbuf->terminal) {
+      errmsg = e_invarg;
+      curbuf->b_p_scbk = -1;
+    } else {
+      if (curbuf->b_p_scbk < -1 || curbuf->b_p_scbk > 100000) {
+        errmsg = e_invarg;
+        curbuf->b_p_scbk = 1000;
+      }
+      // Force the scrollback to take effect.
+      terminal_resize(curbuf->terminal, UINT16_MAX, UINT16_MAX);
+    }
   }
 
   /*
@@ -5426,6 +5433,7 @@ static char_u *get_varp(vimoption_T *p)
   case PV_PI:     return (char_u *)&(curbuf->b_p_pi);
   case PV_QE:     return (char_u *)&(curbuf->b_p_qe);
   case PV_RO:     return (char_u *)&(curbuf->b_p_ro);
+  case PV_SCBK:   return (char_u *)&(curbuf->b_p_scbk);
   case PV_SI:     return (char_u *)&(curbuf->b_p_si);
   case PV_STS:    return (char_u *)&(curbuf->b_p_sts);
   case PV_SUA:    return (char_u *)&(curbuf->b_p_sua);
@@ -5636,6 +5644,7 @@ void buf_copy_options(buf_T *buf, int flags)
       buf->b_p_ai = p_ai;
       buf->b_p_ai_nopaste = p_ai_nopaste;
       buf->b_p_sw = p_sw;
+      buf->b_p_scbk = -1;
       buf->b_p_tw = p_tw;
       buf->b_p_tw_nopaste = p_tw_nopaste;
       buf->b_p_tw_nobin = p_tw_nobin;
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index d077f5f2f7..9c6393e014 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -739,6 +739,7 @@ enum {
   , BV_PI
   , BV_QE
   , BV_RO
+  , BV_SCBK
   , BV_SI
   , BV_SMC
   , BV_SYN
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 4130e69858..e12860c0cc 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -1913,6 +1913,14 @@ return {
       defaults={if_true={vi=12}}
     },
     {
+      full_name='scrollback', abbreviation='scbk',
+      type='number', scope={'buffer'},
+      vi_def=true,
+      varname='p_scbk',
+      redraw={'current_buffer'},
+      defaults={if_true={vi=-1}}
+    },
+    {
       full_name='scrollbind', abbreviation='scb',
       type='bool', scope={'window'},
       vi_def=true,
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 6df443754b..f981fcb875 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -7113,8 +7113,9 @@ void showruler(int always)
   }
   if ((*p_stl != NUL || *curwin->w_p_stl != NUL) && curwin->w_status_height) {
     redraw_custom_statusline(curwin);
-  } else
+  } else {
     win_redr_ruler(curwin, always);
+  }
 
   if (need_maketitle
       || (p_icon && (stl_syntax & STL_IN_ICON))
diff --git a/src/nvim/terminal.c b/src/nvim/terminal.c
index 02500a407c..3fd2814070 100644
--- a/src/nvim/terminal.c
+++ b/src/nvim/terminal.c
@@ -1,18 +1,17 @@
-// VT220/xterm-like terminal emulator implementation for nvim. Powered by
-// libvterm (http://www.leonerd.org.uk/code/libvterm/).
+// VT220/xterm-like terminal emulator.
+// Powered by libvterm http://www.leonerd.org.uk/code/libvterm
 //
 // libvterm is a pure C99 terminal emulation library with abstract input and
 // display. This means that the library needs to read data from the master fd
 // and feed VTerm instances, which will invoke user callbacks with screen
 // update instructions that must be mirrored to the real display.
 //
-// Keys are pressed in VTerm instances by calling
+// Keys are sent to VTerm instances by calling
 // vterm_keyboard_key/vterm_keyboard_unichar, which generates byte streams that
 // must be fed back to the master fd.
 //
-// This implementation uses nvim buffers as the display mechanism for both
-// the visible screen and the scrollback buffer. When focused, the window
-// "pins" to the bottom of the buffer and mirrors libvterm screen state.
+// Nvim buffers are used as the display mechanism for both the visible screen
+// and the scrollback buffer.
 //
 // When a line becomes invisible due to a decrease in screen height or because
 // a line was pushed up during normal terminal output, we store the line
@@ -23,18 +22,17 @@
 // scrollback buffer, which is mirrored in the nvim buffer displaying lines
 // that were previously invisible.
 //
-// The vterm->nvim synchronization is performed in intervals of 10
-// milliseconds. This is done to minimize screen updates when receiving large
-// bursts of data.
+// The vterm->nvim synchronization is performed in intervals of 10 milliseconds,
+// to minimize screen updates when receiving large bursts of data.
 //
 // This module is decoupled from the processes that normally feed it data, so
 // it's possible to use it as a general purpose console buffer (possibly as a
 // log/display mechanism for nvim in the future)
 //
-// Inspired by vimshell (http://www.wana.at/vimshell/) and
-// Conque (https://code.google.com/p/conque/). Libvterm usage instructions (plus
-// some extra code) were taken from
-// pangoterm (http://www.leonerd.org.uk/code/pangoterm/)
+// Inspired by: vimshell http://www.wana.at/vimshell
+//              Conque https://code.google.com/p/conque
+// Some code from pangoterm http://www.leonerd.org.uk/code/pangoterm
+
 #include <assert.h>
 #include <stdio.h>
 #include <stdint.h>
@@ -87,10 +85,10 @@ typedef struct terminal_state {
 # include "terminal.c.generated.h"
 #endif
 
-#define SCROLLBACK_BUFFER_DEFAULT_SIZE 1000
+#define SB_MAX 100000  // Maximum 'scrollback' value.
+
 // Delay for refreshing the terminal buffer after receiving updates from
-// libvterm. This is greatly improves performance when receiving large bursts
-// of data.
+// libvterm. Improves performance when receiving large bursts of data.
 #define REFRESH_DELAY 10
 
 static TimeWatcher refresh_timer;
@@ -102,27 +100,23 @@ typedef struct {
 } ScrollbackLine;
 
 struct terminal {
-  // options passed to terminal_open
-  TerminalOptions opts;
-  // libvterm structures
+  TerminalOptions opts;  // options passed to terminal_open
   VTerm *vt;
   VTermScreen *vts;
   // buffer used to:
   //  - convert VTermScreen cell arrays into utf8 strings
   //  - receive data from libvterm as a result of key presses.
   char textbuf[0x1fff];
-  // Scrollback buffer storage for libvterm.
-  // TODO(tarruda): Use a doubly-linked list
-  ScrollbackLine **sb_buffer;
-  // number of rows pushed to sb_buffer
-  size_t sb_current;
-  // sb_buffer size;
-  size_t sb_size;
+
+  ScrollbackLine **sb_buffer;       // Scrollback buffer storage for libvterm
+  size_t sb_current;                // number of rows pushed to sb_buffer
+  size_t sb_size;                   // sb_buffer size
   // "virtual index" that points to the first sb_buffer row that we need to
   // push to the terminal buffer when refreshing the scrollback. When negative,
   // it actually points to entries that are no longer in sb_buffer (because the
   // window height has increased) and must be deleted from the terminal buffer
   int sb_pending;
+
   // buf_T instance that acts as a "drawing surface" for libvterm
   // we can't store a direct reference to the buffer because the
   // refresh_timer_cb may be called after the buffer was freed, and there's
@@ -130,20 +124,18 @@ struct terminal {
   handle_T buf_handle;
   // program exited
   bool closed, destroy;
+
   // some vterm properties
   bool forward_mouse;
-  // invalid rows libvterm screen
-  int invalid_start, invalid_end;
+  int invalid_start, invalid_end;   // invalid rows in libvterm screen
   struct {
     int row, col;
     bool visible;
   } cursor;
-  // which mouse button is pressed
-  int pressed_button;
-  // pending width/height
-  bool pending_resize;
-  // With a reference count of 0 the terminal can be freed.
-  size_t refcount;
+  int pressed_button;               // which mouse button is pressed
+  bool pending_resize;              // pending width/height
+
+  size_t refcount;                  // reference count
 };
 
 static VTermScreenCallbacks vterm_screen_callbacks = {
@@ -237,25 +229,22 @@ Terminal *terminal_open(TerminalOptions opts)
   rv->invalid_end = opts.height;
   refresh_screen(rv, curbuf);
   set_option_value((uint8_t *)"buftype", 0, (uint8_t *)"terminal", OPT_LOCAL);
-  // some sane settings for terminal buffers
+
+  // Default settings for terminal buffers
+  curbuf->b_p_ma = false;   // 'nomodifiable'
+  curbuf->b_p_ul = -1;      // disable undo
+  curbuf->b_p_scbk = 1000;  // 'scrollback'
   set_option_value((uint8_t *)"wrap", false, NULL, OPT_LOCAL);
   set_option_value((uint8_t *)"number", false, NULL, OPT_LOCAL);
   set_option_value((uint8_t *)"relativenumber", false, NULL, OPT_LOCAL);
   buf_set_term_title(curbuf, (char *)curbuf->b_ffname);
   RESET_BINDING(curwin);
-  // Apply TermOpen autocmds so the user can configure the terminal
+
+  // Apply TermOpen autocmds _before_ configuring the scrollback buffer.
   apply_autocmds(EVENT_TERMOPEN, NULL, NULL, false, curbuf);
 
-  // Configure the scrollback buffer. Try to get the size from:
-  //
-  // - b:terminal_scrollback_buffer_size
-  // - g:terminal_scrollback_buffer_size
-  // - SCROLLBACK_BUFFER_DEFAULT_SIZE
-  //
-  // but limit to 100k.
-  int size = get_config_int("terminal_scrollback_buffer_size");
-  rv->sb_size = size > 0 ? (size_t)size : SCROLLBACK_BUFFER_DEFAULT_SIZE;
-  rv->sb_size = MIN(rv->sb_size, 100000);
+  // Configure the scrollback buffer.
+  rv->sb_size = curbuf->b_p_scbk < 0 ? SB_MAX : (size_t)curbuf->b_p_scbk;;
   rv->sb_buffer = xmalloc(sizeof(ScrollbackLine *) * rv->sb_size);
 
   if (!true_color) {
@@ -334,22 +323,22 @@ void terminal_close(Terminal *term, char *msg)
 void terminal_resize(Terminal *term, uint16_t width, uint16_t height)
 {
   if (term->closed) {
-    // will be called after exited if two windows display the same terminal and
-    // one of the is closed as a consequence of pressing a key.
+    // If two windows display the same terminal and one is closed by keypress.
     return;
   }
+  bool force = width == UINT16_MAX || height == UINT16_MAX;
   int curwidth, curheight;
   vterm_get_size(term->vt, &curheight, &curwidth);
 
-  if (!width) {
+  if (force || !width) {
     width = (uint16_t)curwidth;
   }
 
-  if (!height) {
+  if (force || !height) {
     height = (uint16_t)curheight;
   }
 
-  if (curheight == height && curwidth == width) {
+  if (!force && curheight == height && curwidth == width) {
     return;
   }
 
@@ -381,8 +370,7 @@ void terminal_enter(void)
   State = TERM_FOCUS;
   mapped_ctrl_c |= TERM_FOCUS;  // Always map CTRL-C to avoid interrupt.
   RedrawingDisabled = false;
-  // go to the bottom when the terminal is focused
-  adjust_topline(s->term, buf, false);
+  adjust_topline(s->term, buf, 0);  // scroll to end
   // erase the unfocused cursor
   invalidate_terminal(s->term, s->term->cursor.row, s->term->cursor.row + 1);
   showmode();
@@ -667,10 +655,15 @@ static int term_bell(void *data)
   return 1;
 }
 
-// the scrollback push/pop handlers were copied almost verbatim from pangoterm
+// Scrollback push handler (from pangoterm).
 static int term_sb_push(int cols, const VTermScreenCell *cells, void *data)
 {
   Terminal *term = data;
+
+  if (!term->sb_size) {
+    return 0;
+  }
+
   // copy vterm cells into sb_buffer
   size_t c = (size_t)cols;
   ScrollbackLine *sbrow = NULL;
@@ -682,10 +675,12 @@ static int term_sb_push(int cols, const VTermScreenCell *cells, void *data)
       xfree(term->sb_buffer[term->sb_current - 1]);
     }
 
+    // Make room at the start by shifting to the right.
     memmove(term->sb_buffer + 1, term->sb_buffer,
         sizeof(term->sb_buffer[0]) * (term->sb_current - 1));
 
   } else if (term->sb_current > 0) {
+    // Make room at the start by shifting to the right.
     memmove(term->sb_buffer + 1, term->sb_buffer,
         sizeof(term->sb_buffer[0]) * term->sb_current);
   }
@@ -695,6 +690,7 @@ static int term_sb_push(int cols, const VTermScreenCell *cells, void *data)
     sbrow->cols = c;
   }
 
+  // New row is added at the start of the storage buffer.
   term->sb_buffer[0] = sbrow;
   if (term->sb_current < term->sb_size) {
     term->sb_current++;
@@ -710,6 +706,11 @@ static int term_sb_push(int cols, const VTermScreenCell *cells, void *data)
   return 1;
 }
 
+/// Scrollback pop handler (from pangoterm).
+///
+/// @param cols
+/// @param cells  VTerm state to update.
+/// @param data   Terminal
 static int term_sb_pop(int cols, VTermScreenCell *cells, void *data)
 {
   Terminal *term = data;
@@ -722,24 +723,24 @@ static int term_sb_pop(int cols, VTermScreenCell *cells, void *data)
     term->sb_pending--;
   }
 
-  // restore vterm state
-  size_t c = (size_t)cols;
   ScrollbackLine *sbrow = term->sb_buffer[0];
   term->sb_current--;
+  // Forget the "popped" row by shifting the rest onto it.
   memmove(term->sb_buffer, term->sb_buffer + 1,
       sizeof(term->sb_buffer[0]) * (term->sb_current));
 
-  size_t cols_to_copy = c;
+  size_t cols_to_copy = (size_t)cols;
   if (cols_to_copy > sbrow->cols) {
     cols_to_copy = sbrow->cols;
   }
 
   // copy to vterm state
   memcpy(cells, sbrow->cells, sizeof(cells[0]) * cols_to_copy);
-  for (size_t col = cols_to_copy; col < c; col++) {
+  for (size_t col = cols_to_copy; col < (size_t)cols; col++) {
     cells[col].chars[0] = 0;
     cells[col].width = 1;
   }
+
   xfree(sbrow);
   pmap_put(ptr_t)(invalidated_terminals, term, NULL);
 
@@ -885,7 +886,7 @@ static bool send_mouse_event(Terminal *term, int c)
 // terminal buffer refresh & misc {{{
 
 
-void fetch_row(Terminal *term, int row, int end_col)
+static void fetch_row(Terminal *term, int row, int end_col)
 {
   int col = 0;
   size_t line_len = 0;
@@ -958,23 +959,23 @@ static void refresh_terminal(Terminal *term)
   buf_T *buf = handle_get_buffer(term->buf_handle);
   bool valid = true;
   if (!buf || !(valid = buf_valid(buf))) {
-    // destroyed by `close_buffer`. Dont do anything else
+    // Destroyed by `close_buffer`. Do not do anything else.
     if (!valid) {
       term->buf_handle = 0;
     }
     return;
   }
-  bool pending_resize = term->pending_resize;
+  long ml_before = buf->b_ml.ml_line_count;
   WITH_BUFFER(buf, {
     refresh_size(term, buf);
     refresh_scrollback(term, buf);
     refresh_screen(term, buf);
     redraw_buf_later(buf, NOT_VALID);
   });
-  adjust_topline(term, buf, pending_resize);
+  long ml_added = buf->b_ml.ml_line_count - ml_before;
+  adjust_topline(term, buf, ml_added);
 }
-// libuv timer callback. This will enqueue on_refresh to be processed as an
-// event.
+// Calls refresh_terminal() on all invalidated_terminals.
 static void refresh_timer_cb(TimeWatcher *watcher, void *data)
 {
   if (exiting) {  // Cannot redraw (requires event loop) during teardown/exit.
@@ -1008,7 +1009,37 @@ static void refresh_size(Terminal *term, buf_T *buf)
   term->opts.resize_cb((uint16_t)width, (uint16_t)height, term->opts.data);
 }
 
-// Refresh the scrollback of a invalidated terminal
+/// Adjusts scrollback storage after 'scrollback' option changed.
+static void on_scrollback_option_changed(Terminal *term, buf_T *buf)
+{
+  const size_t scbk = curbuf->b_p_scbk < 0
+                      ? SB_MAX : (size_t)MAX(1, curbuf->b_p_scbk);
+  assert(term->sb_current < SIZE_MAX);
+  if (term->sb_pending > 0) {  // Pending rows must be processed first.
+    abort();
+  }
+
+  // Delete lines exceeding the new 'scrollback' limit.
+  if (scbk < term->sb_current) {
+    size_t diff = term->sb_current - scbk;
+    for (size_t i = 0; i < diff; i++) {
+      ml_delete(1, false);
+      term->sb_current--;
+      xfree(term->sb_buffer[term->sb_current]);
+    }
+    deleted_lines(1, (long)diff);
+  }
+
+  // Resize the scrollback storage.
+  size_t sb_region = sizeof(ScrollbackLine *) * scbk;
+  if (scbk != term->sb_size) {
+    term->sb_buffer = xrealloc(term->sb_buffer, sb_region);
+  }
+
+  term->sb_size = scbk;
+}
+
+// Refresh the scrollback of an invalidated terminal.
 static void refresh_scrollback(Terminal *term, buf_T *buf)
 {
   int width, height;
@@ -1037,9 +1068,11 @@ static void refresh_scrollback(Terminal *term, buf_T *buf)
     ml_delete(buf->b_ml.ml_line_count, false);
     deleted_lines(buf->b_ml.ml_line_count, 1);
   }
+
+  on_scrollback_option_changed(term, buf);
 }
 
-// Refresh the screen(visible part of the buffer when the terminal is
+// Refresh the screen (visible part of the buffer when the terminal is
 // focused) of a invalidated terminal
 static void refresh_screen(Terminal *term, buf_T *buf)
 {
@@ -1048,8 +1081,7 @@ static void refresh_screen(Terminal *term, buf_T *buf)
   int height;
   int width;
   vterm_get_size(term->vt, &height, &width);
-  // It's possible that the terminal height decreased and `term->invalid_end`
-  // doesn't reflect it yet
+  // Terminal height may have decreased before `invalid_end` reflects it.
   term->invalid_end = MIN(term->invalid_end, height);
 
   for (int r = term->invalid_start, linenr = row_to_linenr(term, r);
@@ -1094,14 +1126,6 @@ static void redraw(bool restore_cursor)
     update_screen(0);
   }
 
-  redraw_statuslines();
-
-  if (need_maketitle) {
-    maketitle();
-  }
-
-  showruler(false);
-
   if (term && is_focused(term)) {
     curwin->w_wrow = term->cursor.row;
     curwin->w_wcol = term->cursor.col + win_col_off(curwin);
@@ -1121,21 +1145,21 @@ static void redraw(bool restore_cursor)
   ui_flush();
 }
 
-static void adjust_topline(Terminal *term, buf_T *buf, bool force)
+static void adjust_topline(Terminal *term, buf_T *buf, long added)
 {
   int height, width;
   vterm_get_size(term->vt, &height, &width);
   FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
     if (wp->w_buffer == buf) {
-      // for every window that displays a terminal, ensure the cursor is in a
-      // valid line
-      wp->w_cursor.lnum = MIN(wp->w_cursor.lnum, buf->b_ml.ml_line_count);
-      if (force || curbuf != buf || is_focused(term)) {
-        // if the terminal is not in the current window or if it's focused,
-        // adjust topline/cursor so the window will "follow" the terminal
-        // output
-        wp->w_cursor.lnum = buf->b_ml.ml_line_count;
+      linenr_T ml_end = buf->b_ml.ml_line_count;
+      bool following = ml_end == wp->w_cursor.lnum + added;  // cursor at end?
+      if (following || (wp == curwin && is_focused(term))) {
+        // "Follow" the terminal output
+        wp->w_cursor.lnum = ml_end;
         set_topline(wp, MAX(wp->w_cursor.lnum - height + 1, 1));
+      } else {
+        // Ensure valid cursor for each window displaying this terminal.
+        wp->w_cursor.lnum = MIN(wp->w_cursor.lnum, ml_end);
       }
     }
   }
@@ -1178,17 +1202,6 @@ static char *get_config_string(char *key)
   return NULL;
 }
 
-static int get_config_int(char *key)
-{
-  Object obj;
-  GET_CONFIG_VALUE(key, obj);
-  if (obj.type == kObjectTypeInteger) {
-    return (int)obj.data.integer;
-  }
-  api_free_object(obj);
-  return 0;
-}
-
 // }}}
 
 // vim: foldmethod=marker
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index 8cedfcb905..c95a795587 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -305,16 +305,12 @@ bool undo_allowed(void)
   return true;
 }
 
-/*
- * Get the undolevle value for the current buffer.
- */
+/// Get the 'undolevels' value for the current buffer.
 static long get_undolevel(void)
 {
-  if (curbuf->terminal) {
-    return -1;
-  }
-  if (curbuf->b_p_ul == NO_LOCAL_UNDOLEVEL)
+  if (curbuf->b_p_ul == NO_LOCAL_UNDOLEVEL) {
     return p_ul;
+  }
   return curbuf->b_p_ul;
 }