[PATCH 1/3] vt: don't make VS16 widening depend on earlier screen contents

[Nicolas Pitre]

From: Nicolas Pitre <npitre@xxxxxxxxxxxx>

VS16 (U+FE0F) widens the preceding character only when that character
can be found in the Unicode screen memory. But that memory is allocated
on demand, when the first double-width character is displayed. Until
then vc_uniscr_getc() returns 0 and the VS16 is ignored.

The same sequence, e.g. U+203C U+FE0F, therefore takes one column on a
console that hasn't displayed any double-width character yet, and two
columns afterwards.

Allocate the Unicode screen memory when a VS16 shows up so the
preceding character is always available. vc_uniscr_check() populates
it from the current screen content, which includes that character.

Fixes: 95b05de0a566 ("vt: properly support zero-width Unicode code points")
Assisted-by: LLM
Signed-off-by: Nicolas Pitre <npitre@xxxxxxxxxxxx>
---
 drivers/tty/vt/vt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 49a140b49eff..335253aec549 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3128,6 +3128,15 @@ static int vc_process_ucs(struct vc_data *vc, int *c, int *tc)
 	}
 
 	/* From here the preceding character, if any, must be single-width. */
+
+	/*
+	 * The Unicode screen memory is allocated only when required. VS16
+	 * needs it to find the character it applies to, so it can't depend
+	 * on whether a double-width character was displayed earlier.
+	 */
+	if (curr_c == UCS_VS16)
+		vc_uniscr_check(vc);
+
 	prev_c = vc_uniscr_getc(vc, -1);
 
 	if (curr_c == UCS_VS16 && prev_c != 0) {
-- 
2.55.0