Join empty lines with only one space in `join_selections` (#8989)

* fix: #8977

fixes the issue that lines with only spaces are getting
joined as well

* reverting some renamings

* improve empty line check

* adding integration test

* reverting code block

* fix conditon check for line end

* applying suggested style

2 files changed, 56 insertions, 10 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index cd053266..e557977a 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -4391,10 +4391,9 @@ fn join_selections_impl(cx: &mut Context, select_space: bool) {
     use movement::skip_while;
     let (view, doc) = current!(cx.editor);
     let text = doc.text();
-    let slice = doc.text().slice(..);
+    let slice = text.slice(..);
 
     let mut changes = Vec::new();
-    let fragment = Tendril::from(" ");
 
     for selection in doc.selection(view.id) {
         let (start, mut end) = selection.line_range(slice);
@@ -4410,9 +4409,13 @@ fn join_selections_impl(cx: &mut Context, select_space: bool) {
             let mut end = text.line_to_char(line + 1);
             end = skip_while(slice, end, |ch| matches!(ch, ' ' | '\t')).unwrap_or(end);
 
-            // need to skip from start, not end
-            let change = (start, end, Some(fragment.clone()));
-            changes.push(change);
+            let separator = if end == line_end_char_index(&slice, line + 1) {
+                // the joining line contains only space-characters => don't include a whitespace when joining
+                None
+            } else {
+                Some(Tendril::from(" "))
+            };
+            changes.push((start, end, separator));
         }
     }
 
@@ -4424,9 +4427,6 @@ fn join_selections_impl(cx: &mut Context, select_space: bool) {
     changes.sort_unstable_by_key(|(from, _to, _text)| *from);
     changes.dedup();
 
-    // TODO: joining multiple empty lines should be replaced by a single space.
-    // need to merge change ranges that touch
-
     // select inserted spaces
     let transaction = if select_space {
         let ranges: SmallVec<_> = changes
@@ -4438,9 +4438,9 @@ fn join_selections_impl(cx: &mut Context, select_space: bool) {
             })
             .collect();
         let selection = Selection::new(ranges, 0);
-        Transaction::change(doc.text(), changes.into_iter()).with_selection(selection)
+        Transaction::change(text, changes.into_iter()).with_selection(selection)
     } else {
-        Transaction::change(doc.text(), changes.into_iter())
+        Transaction::change(text, changes.into_iter())
     };
 
     doc.apply(&transaction, view.id);
diff --git a/helix-term/tests/test/commands.rs b/helix-term/tests/test/commands.rs
index b3e13551..e52b142c 100644
--- a/helix-term/tests/test/commands.rs
+++ b/helix-term/tests/test/commands.rs
@@ -480,3 +480,49 @@ fn bar() {#(\n|)#\
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread")]
+async fn test_join_selections() -> anyhow::Result<()> {
+    // normal join
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+            def
+        "}),
+        "J",
+        platform_line(indoc! {"\
+            #[a|]#bc def
+        "}),
+    ))
+    .await?;
+
+    // join with empty line
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+
+            def
+        "}),
+        "JJ",
+        platform_line(indoc! {"\
+            #[a|]#bc def
+        "}),
+    ))
+    .await?;
+
+    // join with additional space in non-empty line
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+
+                def
+        "}),
+        "JJ",
+        platform_line(indoc! {"\
+            #[a|]#bc def
+        "}),
+    ))
+    .await?;
+
+    Ok(())
+}