From 7b463ea6eb54eb5e98e67e77f033c266a575f3c2 Mon Sep 17 00:00:00 2001
From: Scott Hebert <scott.hebert@gmail.com>
Date: Thu, 20 Aug 2026 09:55:56 -0700
Subject: [PATCH] fix: scroll offset_x by visual width, not char count, in
 focus()
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focus() compared the cursor's raw character-count column against the
viewport width when deciding whether to scroll horizontally. For lines
with double-width glyphs (CJK, emoji), character count under-estimates
the true display width, so focus() could decide no scroll was needed
while the cursor was actually off-screen — disagreeing with
get_visible_cursor(), which already computes visual columns correctly
via grapheme widths.

focus() now computes cursor/offset_x visual columns the same way
get_visible_cursor() does, and resolves scroll targets back to a char
index via a new char_idx_for_visual_col() helper.

Fixes #15
---
 src/editor.rs  | 50 +++++++++++++++++++++++++++++++++++++++++++++-----
 tests/input.rs | 13 +++++++++++++
 2 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/src/editor.rs b/src/editor.rs
index cec967a..9646e8c 100644
--- a/src/editor.rs
+++ b/src/editor.rs
@@ -151,16 +151,34 @@ impl Editor {
         let line_number_width = self.get_line_number_width();
 
         let line = self.code.char_to_line(self.cursor);
-        let col = self.cursor - self.code.line_to_char(line);
+        let line_start_char = self.code.line_to_char(line);
+        let line_len = self.code.line_len(line);
+        let col = self.cursor - line_start_char;
 
         let visible_width = width.saturating_sub(line_number_width);
         let visible_height = height;
 
         let step_size = 10;
-        if col < self.offset_x {
-            self.offset_x = col.saturating_sub(step_size);
-        } else if col >= self.offset_x + visible_width {
-            self.offset_x = col.saturating_sub(visible_width.saturating_sub(step_size));
+
+        let visual_col_of = |char_col: usize| -> usize {
+            let slice = self
+                .code
+                .char_slice(line_start_char, line_start_char + char_col.min(line_len));
+            RopeGraphemes::new(&slice).map(grapheme_width).sum()
+        };
+
+        let cursor_visual_col = visual_col_of(col);
+        let offset_visual_col = visual_col_of(self.offset_x);
+
+        if cursor_visual_col < offset_visual_col {
+            let target_visual_col = cursor_visual_col.saturating_sub(step_size);
+            self.offset_x =
+                Self::char_idx_for_visual_col(&self.code, line_start_char, line_len, target_visual_col);
+        } else if cursor_visual_col >= offset_visual_col + visible_width {
+            let target_visual_col =
+                cursor_visual_col.saturating_sub(visible_width.saturating_sub(step_size));
+            self.offset_x =
+                Self::char_idx_for_visual_col(&self.code, line_start_char, line_len, target_visual_col);
         }
 
         let visual_line = self.visual_line_idx(line);
@@ -175,6 +193,28 @@ impl Editor {
         }
     }
 
+    /// Finds the char index (relative to `line_start_char`) whose cumulative grapheme
+    /// display width first reaches `target_visual_col`, so it can be assigned to `offset_x`.
+    fn char_idx_for_visual_col(
+        code: &Code,
+        line_start_char: usize,
+        line_len: usize,
+        target_visual_col: usize,
+    ) -> usize {
+        let slice = code.char_slice(line_start_char, line_start_char + line_len);
+        let mut visual_col = 0;
+        let mut char_idx = 0;
+        for g in RopeGraphemes::new(&slice) {
+            if visual_col >= target_visual_col {
+                break;
+            }
+            let (g_width, g_chars) = grapheme_width_and_chars_len(g);
+            visual_col += g_width;
+            char_idx += g_chars;
+        }
+        char_idx
+    }
+
     /// Handles a mouse button press at the given cursor position, updating selection and click state.
     pub fn handle_mouse_down(&mut self, cursor: usize) {
         let kind = self.clicks.register(cursor);
diff --git a/tests/input.rs b/tests/input.rs
index 7e13a06..e73ee1b 100644
--- a/tests/input.rs
+++ b/tests/input.rs
@@ -45,3 +45,16 @@ fn toggle_fold_at_cursor_toggles_cursor_line_fold() {
     editor.set_cursor(source.find("value").unwrap());
     assert!(editor.get_visible_cursor(&area).is_none());
 }
+
+#[test]
+fn focus_scrolls_horizontally_for_wide_glyph_lines() {
+    let line: String = std::iter::repeat('界').take(20).collect();
+    let mut editor = Editor::new("text", &line, vec![]).unwrap();
+    let area = Rect::new(0, 0, 40, 10);
+
+    editor.set_cursor(line.chars().count());
+    editor.focus(&area);
+
+    assert!(editor.get_offset_x() > 0);
+    assert!(editor.get_visible_cursor(&area).is_some());
+}