fix: Crashing due to input element hack (#36461)

This PR addresses a crash in text input element due to a hack to prevent
text input being trimmed.

The updated behavior will only add `zero width space` unicode character
to the node if there's no text content instead of adding it constantly.
Also by adding the same `zero width space` unicode character to text
area when there's no text content within will allow text area element to
properly display caret.

<img width="197" alt="截圖 2025-04-11 中午12 51 00"
src="https://github.com/user-attachments/assets/10bc7314-9aa3-49df-baac-ef26d39a96d8"
/>

This PR also addresses issues with multiple glyph runs:


https://github.com/user-attachments/assets/658db56f-b166-47ec-bc77-c6c13114d669

Testing: This PR is tested using:
- `./mach run -d 'data:text/html,<input id="input_element"
value="xxxxxxxxxxxxxxxxxxxx">'`
- `./mach run -d 'data:text/html,<textarea id="input_element"
value="xxxxxxxxxxxxxxxxxxxx">'`

without causing crashes, while the results should be covered by WPT
tests

Fixes: https://github.com/servo/servo/issues/36449

---------

Signed-off-by: DK Liao <dklassic@gmail.com>

components/layout_2020/dom_traversal.rs

@@ -212,12 +212,17 @@ fn traverse_children_of<'dom, Node>(
     if is_text_input_element || is_textarea_element {
         let info = NodeAndStyleInfo::new(parent_element, parent_element.style(context));
 
-        if is_text_input_element {
+        if parent_element
+            .to_threadsafe()
+            .node_text_content()
+            .is_empty()
+        {
             // The addition of zero-width space here forces the text input to have an inline formatting
             // context that might otherwise be trimmed if there's no text. This is important to ensure
             // that the input element is at least as tall as the line gap of the caret:
             // <https://drafts.csswg.org/css-ui/#element-with-default-preferred-size>.
             //
+            // This is also used to ensure that the caret will still be rendered when the input is empty.
             // TODO: Is there a less hacky way to do this?
             handler.handle_text(&info, "\u{200B}".into());
         }