From d7742d031276a19160dc9317ff6fb5aac861d853 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Fri, 12 Jun 2026 15:34:12 +0200
Subject: [PATCH] term: do not allow codepoint merging into grapheme clusters
 directly after a cursor move

That is, only do grapheme clustering when printing codepoints in
sequence, without any cursor movements in between.

Closes #2383
---
 CHANGELOG.md |  7 +++++++
 terminal.c   | 13 ++++++++++++-
 terminal.h   |  2 ++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 16bf715d..803408af 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -73,6 +73,13 @@
 ## Unreleased
 ### Added
 ### Changed
+
+* Do not allow codepoints to be merged into grapheme clusters directly
+  after a cursor move ([#2383][2383]).
+
+[2383]: https://codeberg.org/dnkl/foot/issues/2383
+
+
 ### Deprecated
 ### Removed
 ### Fixed
diff --git a/terminal.c b/terminal.c
index 8eafbcbe..374d4371 100644
--- a/terminal.c
+++ b/terminal.c
@@ -4086,6 +4086,10 @@ term_print(struct terminal *term, char32_t wc, int width, bool insert_mode_disab
         xassert(!grid->cursor.lcf);
 
     grid->cursor.point.col = col;
+
+#if defined(FOOT_GRAPHEME_CLUSTERING)
+    term->vt.codepoint_merging_ok = true;
+#endif
 }
 
 static void
@@ -4126,6 +4130,9 @@ ascii_printer_fast(struct terminal *term, char32_t wc)
         xassert(!grid->cursor.lcf);
 
     grid->cursor.point.col = col;
+#if defined(FOOT_GRAPHEME_CLUSTERING)
+    term->vt.codepoint_merging_ok = true;
+#endif
 
     if (unlikely(row->extra != NULL)) {
         grid_row_uri_range_erase(row, uri_start, uri_start);
@@ -4210,7 +4217,11 @@ term_process_and_print_non_ascii(struct terminal *term, char32_t wc)
 {
     int width = c32width(wc);
     bool insert_mode_disable = false;
-    const bool grapheme_clustering = term->grapheme_shaping;
+    const bool grapheme_clustering = term->grapheme_shaping
+#if defined(FOOT_GRAPHEME_CLUSTERING)
+        && term->vt.codepoint_merging_ok
+#endif
+        ;
 
 #if !defined(FOOT_GRAPHEME_CLUSTERING)
     xassert(!grapheme_clustering);
diff --git a/terminal.h b/terminal.h
index 446d5f23..7e95697e 100644
--- a/terminal.h
+++ b/terminal.h
@@ -264,6 +264,7 @@ struct vt {
     char32_t last_printed;
 #if defined(FOOT_GRAPHEME_CLUSTERING)
     utf8proc_int32_t grapheme_state;
+    bool codepoint_merging_ok;
 #endif
     char32_t utf8;
     struct {
@@ -994,5 +995,6 @@ static inline void term_reset_grapheme_state(struct terminal *term)
 {
 #if defined(FOOT_GRAPHEME_CLUSTERING)
     term->vt.grapheme_state = 0;
+    term->vt.codepoint_merging_ok = false;
 #endif
 }