From a7fce9353ad753a9914ec96fb2148603c8c74661 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sat, 10 Dec 2016 14:18:27 +0530
Subject: [PATCH] Fix an off-by-one error when scrolling back

---
 kitty/char_grid.py | 8 ++++----
 kitty/history.c    | 2 +-
 kitty/screen.c     | 2 +-
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/kitty/char_grid.py b/kitty/char_grid.py
index 5bc44db2f..9120e9228 100644
--- a/kitty/char_grid.py
+++ b/kitty/char_grid.py
@@ -183,7 +183,7 @@ class Selection:  # {{{
 
         def line(y):
             if y < 0:
-                return historybuf.line(-y)
+                return historybuf.line(-1 - y)
             return linebuf.line(y)
 
         lines = []
@@ -286,7 +286,7 @@ class CharGrid:
             amt = {'line': 1, 'page': self.screen.lines - 1, 'full': self.screen.historybuf.count}[amt]
         if not upwards:
             amt *= -1
-        y = max(0, min(self.scrolled_by + amt, self.screen.historybuf.count - 1))
+        y = max(0, min(self.scrolled_by + amt, self.screen.historybuf.count))
         if y != self.scrolled_by:
             self.scrolled_by = y
             self.update_cell_data()
@@ -298,7 +298,7 @@ class CharGrid:
             cursor_changed, history_line_added_count = self.screen.update_cell_data(
                 sprites.backend, self.color_profile, addressof(self.main_sprite_map), self.default_fg, self.default_bg, force_full_refresh)
             if self.scrolled_by:
-                self.scrolled_by = min(self.scrolled_by + history_line_added_count, self.screen.historybuf.count - 1)
+                self.scrolled_by = min(self.scrolled_by + history_line_added_count, self.screen.historybuf.count)
                 self.screen.set_scroll_cell_data(
                     sprites.backend, self.color_profile, addressof(self.main_sprite_map), self.default_fg, self.default_bg,
                     self.scrolled_by, addressof(self.scroll_sprite_map))
@@ -370,7 +370,7 @@ class CharGrid:
         if y >= 0 and y < self.screen.lines:
             if self.scrolled_by:
                 if y < self.scrolled_by:
-                    return self.screen.historybuf.line(self.scrolled_by - y)
+                    return self.screen.historybuf.line(self.scrolled_by - 1 - y)
                 return self.screen.line(y - self.scrolled_by)
             else:
                 return self.screen.line(y)
diff --git a/kitty/history.c b/kitty/history.c
index 000570305..3af7f66de 100644
--- a/kitty/history.c
+++ b/kitty/history.c
@@ -130,7 +130,7 @@ line(HistoryBuf *self, PyObject *val) {
 #define line_doc "Return the line with line number val. This buffer grows upwards, i.e. 0 is the most recently added line"
     if (self->count == 0) { PyErr_SetString(PyExc_IndexError, "This buffer is empty"); return NULL; }
     index_type lnum = PyLong_AsUnsignedLong(val);
-    if (lnum > self->count - 1) { PyErr_SetString(PyExc_IndexError, "Out of bounds"); return NULL; }
+    if (lnum >= self->count) { PyErr_SetString(PyExc_IndexError, "Out of bounds"); return NULL; }
     init_line(self, index_of(self, lnum), self->line);
     Py_INCREF(self->line);
     return (PyObject*)self->line;
diff --git a/kitty/screen.c b/kitty/screen.c
index 82b2d565b..ec8144a1c 100644
--- a/kitty/screen.c
+++ b/kitty/screen.c
@@ -1080,7 +1080,7 @@ set_scroll_cell_data(Screen *self, PyObject *args) {
     scrolled_by = MIN(self->historybuf->count, scrolled_by);
 
     for (index_type y = 0; y < MIN(self->lines, scrolled_by); y++) {
-        historybuf_init_line(self->historybuf, scrolled_by - y, self->historybuf->line);
+        historybuf_init_line(self->historybuf, scrolled_by - 1 - y, self->historybuf->line);
         self->historybuf->line->ynum = y;
         if (!update_cell_range_data(&(self->modes), spm, self->historybuf->line, 0, self->columns - 1, color_profile, default_bg, default_fg, data)) return NULL;
     }