From 66d9b8da604e4fd3aa6ee96da3609ff6b9896f50 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Fri, 16 Jun 2023 16:20:37 +0200
Subject: [PATCH] sixel: fix cursor positioning logic
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This adjusts the logic that positions the text cursor after emitting a
sixel, when sixel scrolling mode is *enabled*.

We’ve always mimicked XTerm’s behavior. However, XTerm recently
changed its behavior, to better match that of an VT382.

Now, the cursor is placed *on* the last row of the sixel, instead of
on a new row after the sixel.

This allows applications to print sixels to the bottom row of the
terminal, without causing the content to scroll.

Finally, there was a bug in the horizontal positioning of the cursor;
it was placed on the *first* column of the row, instead of on the
first column of the sixel.
---
 CHANGELOG.md |  9 +++++++++
 sixel.c      | 37 +++++++++++++++++--------------------
 2 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 658e0c1d..a2fc9ec8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -65,6 +65,15 @@
 * Opaque sixels now retain the background opacity (when current
   background color is the **default** background color)
   ([#1360][1360]).
+* Text cursor’s vertical position after emitting a sixel, when sixel
+  scrolling is **enabled** (the default) has been updated to match
+  XTerm’s, and the VT382’s behavior: the cursor is positioned **on**
+  the last sixel row, rather than _after_ it. This allows printing
+  sixels on the last row without scrolling up, but also means
+  applications may have to explicitly emit a newline to ensure the
+  sixel is visible. For example, `cat`:ing a sixel in the shell will
+  typically result in the last row not being visible, unless a newline
+  is explicitly added.
 
 [1371]: https://codeberg.org/dnkl/foot/pulls/1371
 [1360]: https://codeberg.org/dnkl/foot/issues/1360
diff --git a/sixel.c b/sixel.c
index 70ec2ee5..1af214a9 100644
--- a/sixel.c
+++ b/sixel.c
@@ -1043,6 +1043,23 @@ sixel_unhook(struct terminal *term)
         pixel_rows_left -= height;
         rows_avail -= image.rows;
 
+        if (do_scroll) {
+             /* Yes, truncate last row. This matches XTerm’s, and VT382’s behavior */
+            const int linefeed_count = image.height / term->cell_height;
+            for (size_t i = 0; i < linefeed_count; i++)
+                term_linefeed(term);
+
+            /* Position text cursor if this is the last image chunk */
+            if (rows_avail == 0) {
+                term_cursor_to(
+                    term,
+                    term->grid->cursor.point.row,
+                    (term->sixel.cursor_right_of_graphics
+                     ? min(image.pos.col + image.cols, term->cols - 1)
+                     : image.pos.col));
+            }
+        }
+
         /* Dirty touched cells, and scroll terminal content if necessary */
         for (size_t i = 0; i < image.rows; i++) {
             struct row *row = term->grid->rows[cur_row + i];
@@ -1055,26 +1072,6 @@ sixel_unhook(struct terminal *term)
                 row->cells[col].attrs.clean = 0;
             }
 
-            if (do_scroll) {
-                /*
-                 * Linefeed, *unless* we're on the very last row of
-                 * the final image (not just this chunk) and private
-                 * mode 8452 (leave cursor at the right of graphics)
-                 * is enabled.
-                 */
-                if (term->sixel.cursor_right_of_graphics &&
-                    rows_avail == 0 &&
-                    i >= image.rows - 1)
-                {
-                    term_cursor_to(
-                        term,
-                        term->grid->cursor.point.row,
-                        min(image.pos.col + image.cols, term->cols - 1));
-                } else {
-                    term_linefeed(term);
-                    term_carriage_return(term);
-                }
-            }
         }
 
         _sixel_overwrite_by_rectangle(