Auto merge of #19946 - paavininanda:Issue18613, r=<try>

Change Offset_to_text_point function and add unit tests

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #(#18613).

<!-- Either: -->
- [x] These changes do not require tests because they solve an issue related to a already existing code

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19946)
<!-- Reviewable:end -->

components/script/textinput.rs

@@ -871,13 +871,12 @@ impl<T: ClipboardProvider> TextInput<T> {
     fn offset_to_text_point(&self, abs_point: usize) -> TextPoint {
         let mut index = abs_point;
         let mut line = 0;
-
         let last_line_idx = self.lines.len() - 1;
         self.lines.iter().enumerate().fold(0, |acc, (i, val)| {
             if i != last_line_idx {
-                let line_end = max(val.len(), 1);
-                let new_acc = acc + line_end;
-                if abs_point > new_acc && index > line_end {
+                let line_end = val.len();
+                let new_acc = acc + line_end + 1;
+                if abs_point >= new_acc && index > line_end {
                     index -= line_end + 1;
                     line += 1;
                 }

tests/unit/script/textinput.rs

@@ -600,6 +600,13 @@ fn test_textinput_set_selection_with_direction() {
     assert!(textinput.selection_origin.is_some());
     assert_eq!(textinput.selection_origin.unwrap().line, 0);
     assert_eq!(textinput.selection_origin.unwrap().index, 6);
+
+    textinput = text_input(Lines::Multiple, "\n\n");
+    textinput.set_selection_range(0, 1, SelectionDirection::Forward);
+    assert_eq!(textinput.edit_point.line, 1);
+    assert_eq!(textinput.edit_point.index, 0);
+    assert_eq!(textinput.selection_direction, SelectionDirection::Forward);
+
 }
 
 #[test]
@@ -629,4 +636,11 @@ fn test_selection_bounds() {
     assert_eq!(TextPoint { line: 0, index: 6 }, textinput.selection_end());
     assert_eq!(3, textinput.selection_start_offset());
     assert_eq!(6, textinput.selection_end_offset());
+
+    textinput = text_input(Lines::Multiple, "\n\n");
+    textinput.set_selection_range(0, 1, SelectionDirection::Forward);
+    assert_eq!(TextPoint { line: 0, index: 0 }, textinput.selection_origin_or_edit_point());
+    assert_eq!(TextPoint { line: 0, index: 0 }, textinput.selection_start());
+    assert_eq!(TextPoint { line: 1, index: 0 }, textinput.selection_end());
+
 }