src/app/state/find.rs

@@ -26,11 +26,33 @@ impl TextEditor {
             };
 
             let mut start = 0;
-            while let Some(pos) = search_content[start..].find(&query) {
-                let absolute_pos = start + pos;
-                self.find_matches
-                    .push((absolute_pos, absolute_pos + query.len()));
-                start = absolute_pos + 1;
+            while start < search_content.len() {
+                let search_slice = if search_content.is_char_boundary(start) {
+                    &search_content[start..]
+                } else {
+                    // Find next valid boundary
+                    while start < search_content.len() && !search_content.is_char_boundary(start) {
+                        start += 1;
+                    }
+                    if start >= search_content.len() {
+                        break;
+                    }
+                    &search_content[start..]
+                };
+                
+                if let Some(pos) = search_slice.find(&query) {
+                    let absolute_pos = start + pos;
+                    self.find_matches
+                        .push((absolute_pos, absolute_pos + query.len()));
+                    
+                    // Advance to next valid character boundary instead of just +1
+                    start = absolute_pos + 1;
+                    while start < search_content.len() && !search_content.is_char_boundary(start) {
+                        start += 1;
+                    }
+                } else {
+                    break;
+                }
             }
 
             if !self.find_matches.is_empty() {
@@ -94,8 +116,8 @@ impl TextEditor {
             if let Some(active_tab) = self.get_active_tab() {
                 let content = &active_tab.content;
 
-                let start_char = content[..start_byte.min(content.len())].chars().count();
-                let end_char = content[..end_byte.min(content.len())].chars().count();
+                let start_char = Self::safe_slice_to_pos(content, start_byte).chars().count();
+                let end_char = Self::safe_slice_to_pos(content, end_byte).chars().count();
 
                 let text_edit_id = egui::Id::new("main_text_editor");
                 if let Some(mut state) = egui::TextEdit::load_state(ctx, text_edit_id) {
@@ -132,8 +154,7 @@ impl TextEditor {
             self.update_find_matches();
 
             if let Some(active_tab) = self.get_active_tab() {
-                let replacement_end_char = active_tab.content
-                    [..replacement_end.min(active_tab.content.len())]
+                let replacement_end_char = Self::safe_slice_to_pos(&active_tab.content, replacement_end)
                     .chars()
                     .count();
 

src/app/state/processing.rs

@@ -2,7 +2,7 @@ use super::editor::{TextEditor, TextProcessingResult};
 use eframe::egui;
 
 impl TextEditor {
-    fn safe_slice_to_pos(content: &str, pos: usize) -> &str {
+    pub(crate) fn safe_slice_to_pos(content: &str, pos: usize) -> &str {
         let pos = pos.min(content.len());
         let mut boundary_pos = pos;
         while boundary_pos > 0 && !content.is_char_boundary(boundary_pos) {

src/ui/central_panel/find_highlight.rs

@@ -1,5 +1,15 @@
 use eframe::egui;
 
+/// Safely get a string slice up to a byte position, ensuring UTF-8 boundaries
+fn safe_slice_to_pos(content: &str, pos: usize) -> &str {
+    let pos = pos.min(content.len());
+    let mut boundary_pos = pos;
+    while boundary_pos > 0 && !content.is_char_boundary(boundary_pos) {
+        boundary_pos -= 1;
+    }
+    &content[..boundary_pos]
+}
+
 pub(super) fn draw_find_highlights(
     ui: &mut egui::Ui,
     content: &str,
@@ -44,7 +54,7 @@ fn draw_single_highlight(
     font_id: &egui::FontId,
     is_current_match: bool,
 ) {
-    let text_up_to_start = &content[..start_pos.min(content.len())];
+    let text_up_to_start = safe_slice_to_pos(content, start_pos);
     let start_line = text_up_to_start.chars().filter(|&c| c == '\n').count();
 
     if start_line >= galley.rows.len() {
@@ -52,8 +62,8 @@ fn draw_single_highlight(
     }
 
     let line_start_byte_pos = text_up_to_start.rfind('\n').map(|pos| pos + 1).unwrap_or(0);
-    let line_start_char_pos = content[..line_start_byte_pos].chars().count();
-    let start_char_pos = content[..start_pos].chars().count();
+    let line_start_char_pos = safe_slice_to_pos(content, line_start_byte_pos).chars().count();
+    let start_char_pos = safe_slice_to_pos(content, start_pos).chars().count();
     let start_col = start_char_pos - line_start_char_pos;
 
     let lines: Vec<&str> = content.lines().collect();