don't move cursor while forward deleting in append mode

Currently, when forward deleting (`delete_char_forward` bound to `del`,
`delete_word_forward`, `kill_to_line_end`) the cursor is moved to the
left in append mode (or generally when the cursor is at the end of the
selection). For example in a document `|abc|def`  (|indicates selection)
if enter append mode the cursor is moved to `c` and the selection
becomes: `|abcd|ef`. When deleting forward (`del`) `d` is deleted. The
expectation would be that the selection doesn't shrink so that `del`
again deletes `e` and then `f`. This would look as follows:

`|abcd|ef`
`|abce|f`
`|abcf|`
`|abc |`

This is inline with how other editors like kakoune work.
However, helix currently moves the selection backwards leading to the
following behavior:

`|abcd|ef`
`|abc|ef`
`|ab|ef`
`ef`

This means that `delete_char_forward` essentially acts like
`delete_char_backward` after deleting the first character in append
mode.

To fix the problem the cursor must be moved to the right while deleting
forward (first fix in this commit). Furthermore, when the EOF char is
reached a newline char must be inserted (just like when entering
appendmode) to prevent the cursor from moving to the right

3 files changed, 124 insertions, 45 deletions

diff --git a/helix-core/src/transaction.rs b/helix-core/src/transaction.rs
index 06efe259..f4f94b54 100644
--- a/helix-core/src/transaction.rs
+++ b/helix-core/src/transaction.rs
@@ -570,6 +570,11 @@ impl Transaction {
         Self::from(changeset)
     }
 
+    pub fn insert_at_eof(mut self, text: Tendril) -> Transaction {
+        self.changes.insert(text);
+        self
+    }
+
     /// Generate a transaction with a change per selection range.
     pub fn change_by_selection<F>(doc: &Rope, selection: &Selection, f: F) -> Self
     where
diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index 05b5b9b6..5a844e35 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -795,42 +795,50 @@ fn extend_to_line_start(cx: &mut Context) {
 }
 
 fn kill_to_line_start(cx: &mut Context) {
-    delete_by_selection_insert_mode(cx, move |text, range| {
-        let line = range.cursor_line(text);
-        let first_char = text.line_to_char(line);
-        let anchor = range.cursor(text);
-        let head = if anchor == first_char && line != 0 {
-            // select until previous line
-            line_end_char_index(&text, line - 1)
-        } else if let Some(pos) = find_first_non_whitespace_char(text.line(line)) {
-            if first_char + pos < anchor {
-                // select until first non-blank in line if cursor is after it
-                first_char + pos
+    delete_by_selection_insert_mode(
+        cx,
+        move |text, range| {
+            let line = range.cursor_line(text);
+            let first_char = text.line_to_char(line);
+            let anchor = range.cursor(text);
+            let head = if anchor == first_char && line != 0 {
+                // select until previous line
+                line_end_char_index(&text, line - 1)
+            } else if let Some(pos) = find_first_non_whitespace_char(text.line(line)) {
+                if first_char + pos < anchor {
+                    // select until first non-blank in line if cursor is after it
+                    first_char + pos
+                } else {
+                    // select until start of line
+                    first_char
+                }
             } else {
                 // select until start of line
                 first_char
-            }
-        } else {
-            // select until start of line
-            first_char
-        };
-        (head, anchor)
-    });
+            };
+            (head, anchor)
+        },
+        Direction::Backward,
+    );
 }
 
 fn kill_to_line_end(cx: &mut Context) {
-    delete_by_selection_insert_mode(cx, |text, range| {
-        let line = range.cursor_line(text);
-        let line_end_pos = line_end_char_index(&text, line);
-        let pos = range.cursor(text);
+    delete_by_selection_insert_mode(
+        cx,
+        |text, range| {
+            let line = range.cursor_line(text);
+            let line_end_pos = line_end_char_index(&text, line);
+            let pos = range.cursor(text);
 
-        // if the cursor is on the newline char delete that
-        if pos == line_end_pos {
-            (pos, text.line_to_char(line + 1))
-        } else {
-            (pos, line_end_pos)
-        }
-    });
+            // if the cursor is on the newline char delete that
+            if pos == line_end_pos {
+                (pos, text.line_to_char(line + 1))
+            } else {
+                (pos, line_end_pos)
+            }
+        },
+        Direction::Forward,
+    );
 }
 
 fn goto_first_nonwhitespace(cx: &mut Context) {
@@ -2322,13 +2330,44 @@ fn delete_selection_impl(cx: &mut Context, op: Operation) {
 fn delete_by_selection_insert_mode(
     cx: &mut Context,
     mut f: impl FnMut(RopeSlice, &Range) -> Deletion,
+    direction: Direction,
 ) {
     let (view, doc) = current!(cx.editor);
     let text = doc.text().slice(..);
-    let transaction =
+    let mut selection = SmallVec::new();
+    let mut insert_newline = false;
+    let text_len = text.len_chars();
+    let mut transaction =
         Transaction::delete_by_selection(doc.text(), doc.selection(view.id), |range| {
-            f(text, range)
+            let (start, end) = f(text, range);
+            if direction == Direction::Forward {
+                let mut range = *range;
+                if range.head > range.anchor {
+                    insert_newline |= end == text_len;
+                    // move the cursor to the right so that the selection
+                    // doesn't shrink when deleting forward (so the text appears to
+                    // move to  left)
+                    // += 1 is enough here as the range is normalized to grapheme boundaries
+                    // later anyway
+                    range.head += 1;
+                }
+                selection.push(range);
+            }
+            (start, end)
         });
+
+    // in case we delete the last character and the cursor would be moved to the EOF char
+    // insert a newline, just like when entering append mode
+    if insert_newline {
+        transaction = transaction.insert_at_eof(doc.line_ending.as_str().into());
+    }
+
+    if direction == Direction::Forward {
+        doc.set_selection(
+            view.id,
+            Selection::new(selection, doc.selection(view.id).primary_index()),
+        );
+    }
     doc.apply(&transaction, view.id);
     lsp::signature_help_impl(cx, SignatureHelpInvoked::Automatic);
 }
@@ -3490,28 +3529,40 @@ pub mod insert {
 
     pub fn delete_char_forward(cx: &mut Context) {
         let count = cx.count();
-        delete_by_selection_insert_mode(cx, |text, range| {
-            let pos = range.cursor(text);
-            (pos, graphemes::nth_next_grapheme_boundary(text, pos, count))
-        })
+        delete_by_selection_insert_mode(
+            cx,
+            |text, range| {
+                let pos = range.cursor(text);
+                (pos, graphemes::nth_next_grapheme_boundary(text, pos, count))
+            },
+            Direction::Forward,
+        )
     }
 
     pub fn delete_word_backward(cx: &mut Context) {
         let count = cx.count();
-        delete_by_selection_insert_mode(cx, |text, range| {
-            let anchor = movement::move_prev_word_start(text, *range, count).from();
-            let next = Range::new(anchor, range.cursor(text));
-            let range = exclude_cursor(text, next, *range);
-            (range.from(), range.to())
-        });
+        delete_by_selection_insert_mode(
+            cx,
+            |text, range| {
+                let anchor = movement::move_prev_word_start(text, *range, count).from();
+                let next = Range::new(anchor, range.cursor(text));
+                let range = exclude_cursor(text, next, *range);
+                (range.from(), range.to())
+            },
+            Direction::Backward,
+        );
     }
 
     pub fn delete_word_forward(cx: &mut Context) {
         let count = cx.count();
-        delete_by_selection_insert_mode(cx, |text, range| {
-            let head = movement::move_next_word_end(text, *range, count).to();
-            (range.cursor(text), head)
-        });
+        delete_by_selection_insert_mode(
+            cx,
+            |text, range| {
+                let head = movement::move_next_word_end(text, *range, count).to();
+                (range.cursor(text), head)
+            },
+            Direction::Forward,
+        );
     }
 }
 
diff --git a/helix-term/tests/test/commands.rs b/helix-term/tests/test/commands.rs
index 1efb204e..f91a6371 100644
--- a/helix-term/tests/test/commands.rs
+++ b/helix-term/tests/test/commands.rs
@@ -429,3 +429,26 @@ async fn test_delete_word_forward() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn test_delete_char_forward() -> anyhow::Result<()> {
+    test((
+        platform_line(indoc! {"\
+                #[abc|]#def
+                #(abc|)#ef
+                #(abc|)#f
+                #(abc|)#
+            "})
+        .as_str(),
+        "a<del><esc>",
+        platform_line(indoc! {"\
+                #[abc|]#ef
+                #(abc|)#f
+                #(abc|)#
+                #(abc|)#
+            "})
+        .as_str(),
+    ))
+    .await?;
+
+    Ok(())
+}