Fix `try_restore_indent` on non-LF documents

On Windows for example the behavior of this function typically diverges
from the usual behavior on Unix. Instead of checking that the inserted
string starts with `'\n'` (untrue for for CRLF line endings) we need to
check that the first grapheme cluster in the string is a line ending.
(All line endings are single grapheme clusters.)

helix-term/tests/test/commands/insert.rs

@@ -512,3 +512,25 @@ async fn test_open_above_with_comments() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread")]
+async fn try_restore_indent() -> anyhow::Result<()> {
+    // Assert that `helix_view::editor::try_restore_indent` handles line endings correctly
+    // endings.
+    test((
+        indoc! {"\
+        if true #[|{]#
+        }
+        "},
+        // `try_restore_indent` should remove the indentation when adding a blank line.
+        ":lang rust<ret>o<esc>",
+        indoc! {"\
+        if true {
+        #[
+        |]#}
+        "},
+    ))
+    .await?;
+
+    Ok(())
+}

helix-view/src/editor.rs

@@ -2262,16 +2262,20 @@ impl Editor {
 
 fn try_restore_indent(doc: &mut Document, view: &mut View) {
     use helix_core::{
-        chars::char_is_whitespace, line_ending::line_end_char_index, Operation, Transaction,
+        chars::char_is_whitespace,
+        line_ending::{line_end_char_index, str_is_line_ending},
+        unicode::segmentation::UnicodeSegmentation,
+        Operation, Transaction,
     };
 
     fn inserted_a_new_blank_line(changes: &[Operation], pos: usize, line_end_pos: usize) -> bool {
         if let [Operation::Retain(move_pos), Operation::Insert(ref inserted_str), Operation::Retain(_)] =
             changes
         {
+            let mut graphemes = inserted_str.graphemes(true);
             move_pos + inserted_str.len() == pos
-                && inserted_str.starts_with('\n')
-                && inserted_str.chars().skip(1).all(char_is_whitespace)
+                && graphemes.next().is_some_and(str_is_line_ending)
+                && graphemes.all(|g| g.chars().all(char_is_whitespace))
                 && pos == line_end_pos // ensure no characters exists after current position
         } else {
             false