tui: Do not optimize left motion at the right margin.

From observation, there are several different possible behaviours:

1. Deferred wrap like a real DEC VT.  The cursor stays visible in the last
   column, and CUB is calculated relative to that column.
   Examples: xterm, Unicode rxvt, PuTTY, nosh console-terminal-emulator,
   FreeBSD kernel's built-in emulator, Linux's built-in emulator
2. Deferred wrap like a real DEC VT.  CUB is calculated relative to the last
   column.  But the cursor is invisible.
   Examples: emulators using newer libvte
3. Non-deferred wrap.  The cursor has already wrapped to the next line and CUB
   does not wrap back.
   Examples: cygwin, Interix
4. Non-deferred wrap that acts like deferred wrap.  The cursor has already
   visibly wrapped to the next line, but CUB can wrap back around the left
   margin.
   Examples: Konsole
5. Deferred wrap with visibly out of bounds cursor.  The cursor visibly moves
   outwith the screen boundaries.  CUB is calculated relative to a cursor
   column that has overflowed the end of the screen grid array.
   Examples: iTerm2
6. Deferred wrap with invisibly out of bounds cursor.  CUB is calculated
   relative to a cursor column that has overflowed the end of the screen grid
   array.  And the cursor is invisible.
   Examples: emulators using older libvte

In many cases, nvim does not have enough information to know which behaviour
the terminal will exhibit, and thus the correct amount of CUB to issue.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index 54893014e9..55ada08663 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -517,16 +517,19 @@ static void cursor_goto(UI *ui, int row, int col)
       }
   }
   if (row == grid->row) {
-    if (col < grid->col) {
+    if (col < grid->col
+        // Deferred right margin wrap terminals have inconsistent ideas about
+        // where the cursor actually is during a deferred wrap.  Relative
+        // motion calculations have OBOEs that cannot be compensated for,
+        // because two terminals that claim to be the same will implement
+        // different cursor positioning rules.
+        && (data->immediate_wrap_after_last_column || grid->col < ui->width)) {
       int n = grid->col - col;
       if (n <= 4) { // This might be just BS, so it is considered really cheap.
         while (n--) {
           unibi_out(ui, unibi_cursor_left);
         }
       } else {
-        if (!data->immediate_wrap_after_last_column && grid->col >= ui->width) {
-          n--;  // We have calculated one too many columns because of delayed wrap.
-        }
         data->params[0].i = n;
         unibi_out(ui, unibi_parm_left_cursor);
       }