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/display_list/mod.rs

@@ -1223,9 +1223,7 @@ fn glyphs(
     glyphs
 }
 
-// TODO: This implementation has not been tested against `TextFragment` with mutiple runs.
-// It is possible that the `glyphs` function above will need to be modified to
-// handle multiple runs correctly.
+// TODO: The implementation here does not account for multiple glyph runs properly.
 fn glyphs_advance_by_index(
     glyph_runs: &[Arc<GlyphStore>],
     index: fonts_traits::ByteIndex,
@@ -1233,11 +1231,13 @@ fn glyphs_advance_by_index(
     justification_adjustment: Au,
 ) -> PhysicalPoint<Au> {
     let mut point = baseline_origin;
+    let mut index = index;
     for run in glyph_runs {
         let total_advance = run.advance_for_byte_range(
-            &ServoRange::new(fonts::ByteIndex(0), index),
+            &ServoRange::new(fonts::ByteIndex(0), index.min(run.len())),
             justification_adjustment,
         );
+        index = index - index.min(run.len());
         point.x += total_advance;
     }
     point