Two fixes for RI codepoints. Firstly, do not combine more than two of

them - previously we were ending up with four codepoints in one cell
which tmux believed to be width 2, but terminals considered width 4.
Secondly, invalidate cursor position before redrawing the cell when the
second codepoint is received, terminals vary in how they manage
backspace and cursor movement across these characters, so it is better
to use absolute rather than relative positioning. GitHub issue 4853.

tty.c

@@ -2051,6 +2051,8 @@ tty_cmd_cell(struct tty *tty, const struct tty_ctx *ctx)
 		tty_region_pane(tty, ctx, ctx->orupper, ctx->orlower);
 
 	tty_margin_off(tty);
+	if (ctx->flags & TTY_CTX_CELL_INVALIDATE)
+		tty_invalidate(tty);
 	tty_cursor_pane_unless_wrap(tty, ctx, ctx->ocx, ctx->ocy);
 
 	tty_cell(tty, ctx->cell, &ctx->defaults, ctx->palette,

utf8-combined.c

@@ -82,6 +82,23 @@ utf8_is_hangul_filler(const struct utf8_data *ud)
 	return (memcmp(ud->data, "\343\205\244", 3) == 0);
 }
 
+/* Count regional indicator characters. */
+static u_int
+utf8_regional_count(const struct utf8_data *ud)
+{
+	u_int	count = 0, i;
+
+	for (i = 0; i + 4 <= ud->size; i++) {
+		if (ud->data[i] == 0xf0 &&
+		    ud->data[i + 1] == 0x9f &&
+		    ud->data[i + 2] == 0x87 &&
+		    ud->data[i + 3] >= 0xa6 &&
+		    ud->data[i + 3] <= 0xbf)
+			count++;
+	}
+	return (count);
+}
+
 /* Should these two characters combine? */
 int
 utf8_should_combine(const struct utf8_data *with, const struct utf8_data *add)
@@ -94,8 +111,13 @@ utf8_should_combine(const struct utf8_data *with, const struct utf8_data *add)
 		return (0);
 
 	/* Regional indicators. */
-	if ((a >= 0x1F1E6 && a <= 0x1F1FF) && (w >= 0x1F1E6 && w <= 0x1F1FF))
+	if ((a >= 0x1F1E6 && a <= 0x1F1FF) && (w >= 0x1F1E6 && w <= 0x1F1FF)) {
+		if (utf8_regional_count(with) != 1)
+			return (0);
+		if (utf8_regional_count(add) != 1)
+			return (0);
 		return (1);
+	}
 
 	/* Emoji skin tone modifiers. */
 	switch (a) {