editor: Fix JoinLines with selection to end-of-line (#48035)

Full line selections should only include the next line if there's a
single line selected. Otherwise, if multiple lines are selected, only
the selected lines should be joined.

Also updates the editor tests to include both of these cases.

Closes #48030 

Release Notes:

- Fixed `editor::JoinLines` when a multi-line selection includes the end
of the final line

crates/editor/src/editor.rs

@@ -11307,6 +11307,15 @@ impl Editor {
             // would do nothing for single line selections individual cursors.
             let end = if selection.start.row == selection.end.row {
                 MultiBufferRow(selection.start.row + 1)
+            } else if selection.end.column == 0 {
+                // If the selection ends at the start of a line, it's logically at the end of the
+                // previous line (plus its newline).
+                // Don't include the end line unless there's only one line selected.
+                if selection.start.row + 1 == selection.end.row {
+                    MultiBufferRow(selection.end.row)
+                } else {
+                    MultiBufferRow(selection.end.row - 1)
+                }
             } else {
                 MultiBufferRow(selection.end.row)
             };

crates/editor/src/editor_tests.rs

@@ -4882,6 +4882,32 @@ fn test_join_lines_with_single_selection(cx: &mut TestAppContext) {
             &[Point::new(0, 3)..Point::new(0, 3)]
         );
 
+        editor.undo(&Undo, window, cx);
+        assert_eq!(buffer.read(cx).text(), "aaa\nbbb\nccc\nddd\n\n");
+
+        // Select a full line, i.e. start of the first line to the start of the second line
+        editor.change_selections(SelectionEffects::no_scroll(), window, cx, |s| {
+            s.select_ranges([Point::new(0, 0)..Point::new(1, 0)])
+        });
+        editor.join_lines(&JoinLines, window, cx);
+        assert_eq!(buffer.read(cx).text(), "aaa bbb\nccc\nddd\n\n");
+
+        editor.undo(&Undo, window, cx);
+        assert_eq!(buffer.read(cx).text(), "aaa\nbbb\nccc\nddd\n\n");
+
+        // Select two full lines
+        editor.change_selections(SelectionEffects::no_scroll(), window, cx, |s| {
+            s.select_ranges([Point::new(0, 0)..Point::new(2, 0)])
+        });
+        editor.join_lines(&JoinLines, window, cx);
+
+        // Only the selected lines should be joined, not the third.
+        assert_eq!(
+            buffer.read(cx).text(),
+            "aaa bbb\nccc\nddd\n\n",
+            "only the two selected lines (a and b) should be joined"
+        );
+
         // When multiple lines are selected, remove newlines that are spanned by the selection
         editor.change_selections(SelectionEffects::no_scroll(), window, cx, |s| {
             s.select_ranges([Point::new(0, 5)..Point::new(2, 2)])