Fix Backspace deleting entire previous line in <textarea> (#36112)

* test(textinput): Add test for backspace at beginning of line in textarea

Introduce a test to reproduce and verify the fix for backspacing at the
beginning of a line in a multiline textarea. This ensures that pressing
Backspace when the cursor is at the start of a line correctly removes the
newline without deleting the entire previous line’s content.

Related to: #27523

Signed-off-by: Emmanuel Elom <elomemmanuel007@gmail.com>

* fix(textinput): Preserve selection origin when adjusting vertical position

Fixes an issue where pressing Backspace at the beginning of a line in a
textarea incorrectly deleted the entire previous line's content. This happened
because `self.adjust_vertical(-1, select)` modified `selection_origin` and
`edit_point`, but `selection_origin` was not restored before performing the
horizontal adjustment. As a result, `self.selection_start()` and
`self.selection_end()` were inconsistent, leading to `replace_operation`
erasing the entire line.

Now, we temporarily store `selection_origin` before adjusting vertical
position and restore it afterward to ensure proper cursor and selection
behavior.

Fixes: #27523
Signed-off-by: Emmanuel Elom <elomemmanuel007@gmail.com>

---------

Signed-off-by: Emmanuel Elom <elomemmanuel007@gmail.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/script/textinput.rs

@@ -671,8 +671,14 @@ impl<T: ClipboardProvider> TextInput<T> {
             Direction::Backward => {
                 let remaining = self.edit_point.index;
                 if adjust > remaining && self.edit_point.line > 0 {
+                    // Preserve the current selection origin because `adjust_vertical`
+                    // modifies `selection_origin`. Since we are moving backward instead of
+                    // highlighting vertically, we need to restore it after adjusting the line.
+                    let selection_origin_temp = self.selection_origin;
                     self.adjust_vertical(-1, select);
                     self.edit_point.index = self.current_line_length();
+                    // Restore the original selection origin to maintain expected behavior.
+                    self.selection_origin = selection_origin_temp;
                     // one shift is consumed by the change of line, hence the -1
                     self.adjust_horizontal(
                         adjust.saturating_sub(remaining + UTF8Bytes::one()),

tests/unit/script/textinput.rs

@@ -856,3 +856,11 @@ fn test_select_all() {
         textinput.selection_end()
     );
 }
+
+#[test]
+fn test_backspace_in_textarea_at_beginning_of_line() {
+    let mut textinput = text_input(Lines::Multiple, "first line\n");
+    textinput.handle_keydown_aux(Key::ArrowDown, Modifiers::empty(), false);
+    textinput.handle_keydown_aux(Key::Backspace, Modifiers::empty(), false);
+    assert_eq!(textinput.get_content(), DOMString::from("first line"));
+}