feat(term): enable reflow by default (#21124)

Problem: Contents of terminal buffer are not reflown when Nvim is
resized.

Solution: Enable reflow in libvterm by default. Now that libvterm is
vendored, also fix "TUI rapid resize" test failures there.

Note: Neovim's scrollback buffer does not support reflow (yet), so lines
vanishing into the buffer due to a too small window will be restored
without reflow.

7 files changed, 19 insertions, 8 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 4df7d6442a..68a05a99a4 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -163,6 +163,9 @@ TERMINAL
   system clipboard (copy). Querying with OSC 52 (paste) is not supported.
 • |hl-StatusLineTerm| and |hl-StatusLineTermNC| define highlights for the
   status line in |terminal| windows.
+• The terminal buffer now supports reflow (wrapped lines adapt when the buffer
+  is resized horizontally). Note: Lines that are not visible and kept in
+  'scrollback' are not reflown.
 
 TREESITTER
 
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index bc2a8ae263..f44e0954a5 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5031,6 +5031,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Minimum is 1, maximum is 100000.
 	Only in |terminal| buffers.
 
+	Note: Lines that are not visible and kept in scrollback are not
+	reflown when the terminal buffer is resized horizontally.
+
 			*'scrollbind'* *'scb'* *'noscrollbind'* *'noscb'*
 'scrollbind' 'scb'	boolean	(default off)
 			local to window
diff --git a/runtime/lua/vim/_meta/options.lua b/runtime/lua/vim/_meta/options.lua
index 10c888548c..b4ac478b61 100644
--- a/runtime/lua/vim/_meta/options.lua
+++ b/runtime/lua/vim/_meta/options.lua
@@ -5253,6 +5253,9 @@ vim.wo.scr = vim.wo.scroll
 --- Minimum is 1, maximum is 100000.
 --- Only in `terminal` buffers.
 ---
+--- Note: Lines that are not visible and kept in scrollback are not
+--- reflown when the terminal buffer is resized horizontally.
+---
 --- @type integer
 vim.o.scrollback = -1
 vim.o.scbk = vim.o.scrollback
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index ef7cc2fb89..3612a80fb8 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -6627,6 +6627,9 @@ return {
         top are deleted if new lines exceed this limit.
         Minimum is 1, maximum is 100000.
         Only in |terminal| buffers.
+
+        Note: Lines that are not visible and kept in scrollback are not
+        reflown when the terminal buffer is resized horizontally.
       ]=],
       full_name = 'scrollback',
       redraw = { 'current_buffer' },
diff --git a/src/nvim/terminal.c b/src/nvim/terminal.c
index abc9b3534b..54a0de9c22 100644
--- a/src/nvim/terminal.c
+++ b/src/nvim/terminal.c
@@ -319,8 +319,7 @@ void terminal_open(Terminal **termpp, buf_T *buf, TerminalOptions opts)
   // Set up screen
   term->vts = vterm_obtain_screen(term->vt);
   vterm_screen_enable_altscreen(term->vts, true);
-  // TODO(clason): reenable when https://github.com/neovim/neovim/issues/23762 is fixed
-  // vterm_screen_enable_reflow(term->vts, true);
+  vterm_screen_enable_reflow(term->vts, true);
   // delete empty lines at the end of the buffer
   vterm_screen_set_callbacks(term->vts, &vterm_screen_callbacks, term);
   vterm_screen_set_unrecognised_fallbacks(term->vts, &vterm_fallbacks, term);
diff --git a/src/vterm/screen.c b/src/vterm/screen.c
index 97a8d50b93..720d1bb939 100644
--- a/src/vterm/screen.c
+++ b/src/vterm/screen.c
@@ -534,7 +534,7 @@ static void resize_buffer(VTermScreen *screen, int bufidx, int new_rows, int new
   while(old_row >= 0) {
     int old_row_end = old_row;
     /* TODO: Stop if dwl or dhl */
-    while(REFLOW && old_lineinfo && old_row >= 0 && old_lineinfo[old_row].continuation)
+    while(REFLOW && old_lineinfo && old_row > 0 && old_lineinfo[old_row].continuation)
       old_row--;
     int old_row_start = old_row;
 
@@ -596,7 +596,7 @@ static void resize_buffer(VTermScreen *screen, int bufidx, int new_rows, int new
 #endif
 
     if(new_row_start < 0) {
-      if(old_row_start <= old_cursor.row && old_cursor.row < old_row_end) {
+      if(old_row_start <= old_cursor.row && old_cursor.row <= old_row_end) {
         new_cursor.row = 0;
         new_cursor.col = old_cursor.col;
         if(new_cursor.col >= new_cols)
diff --git a/test/functional/terminal/window_spec.lua b/test/functional/terminal/window_spec.lua
index 64534ca8dc..18477fdf2d 100644
--- a/test/functional/terminal/window_spec.lua
+++ b/test/functional/terminal/window_spec.lua
@@ -87,7 +87,7 @@ describe(':terminal window', function()
         {7:       1 }tty ready                                |
         {7:       2 }rows: 6, cols: 48                        |
         {7:       3 }abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNO|
-        {7:       4 }WXYZrows: 6, cols: 41                    |
+        {7:       4 }PQRSTUVWXYZrows: 6, cols: 41             |
         {7:       5 }{1: }                                        |
         {7:       6 }                                         |
         {3:-- TERMINAL --}                                    |
@@ -97,7 +97,7 @@ describe(':terminal window', function()
         {7:       1 }tty ready                                |
         {7:       2 }rows: 6, cols: 48                        |
         {7:       3 }abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNO|
-        {7:       4 }WXYZrows: 6, cols: 41                    |
+        {7:       4 }PQRSTUVWXYZrows: 6, cols: 41             |
         {7:       5 } abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMN|
         {7:       6 }OPQRSTUVWXYZ{1: }                            |
         {3:-- TERMINAL --}                                    |
@@ -132,9 +132,9 @@ describe(':terminal window', function()
       screen:expect([[
         {7:++ 7  }                                            |
         {7:++ 8  }abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQR|
-        {7:++ 9  }TUVWXYZ                                     |
+        {7:++ 9  }STUVWXYZ                                    |
         {7:++10  }abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQR|
-        {7:++11  }TUVWXYZrows: 6, cols: 44                    |
+        {7:++11  }STUVWXYZrows: 6, cols: 44                   |
         {7:++12  }{1: }                                           |
         {3:-- TERMINAL --}                                    |
       ]])