Auto merge of #12388 - kilobtye:textarea2, r=pcwalton

Let textarea wrap lines

<!-- Please describe your changes on the following line: -->
1. Change textarea's style from white-space: pre to white-space: pre-wrap.
2. Display insertion point when a line is wrapped.

---
<!-- 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 #10053 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/layout/fragment.rs

@@ -882,16 +882,23 @@ impl Fragment {
         let size = LogicalSize::new(self.style.writing_mode,
                                     split.inline_size,
                                     self.border_box.size.block);
-        let flags = match self.specific {
-            SpecificFragmentInfo::ScannedText(ref info) => info.flags,
-            _ => ScannedTextFlags::empty()
+        // Preserve the insertion point if it is in this fragment's range or it is at line end.
+        let (flags, insertion_point) = match self.specific {
+            SpecificFragmentInfo::ScannedText(ref info) => {
+                match info.insertion_point {
+                    Some(index) if split.range.contains(index) => (info.flags, info.insertion_point),
+                    Some(index) if index == ByteIndex(text_run.text.chars().count() as isize - 1) &&
+                        index == split.range.end() => (info.flags, info.insertion_point),
+                    _ => (info.flags, None)
+                }
+            },
+            _ => (ScannedTextFlags::empty(), None)
         };
-        // FIXME(pcwalton): This should modify the insertion point as necessary.
         let info = box ScannedTextFragmentInfo::new(
             text_run,
             split.range,
             size,
-            None,
+            insertion_point,
             flags);
         self.transform(size, SpecificFragmentInfo::ScannedText(info))
     }