Minor comment fix and add TODO. (flutter#7158)

Author: GaryQian

third_party/txt/src/txt/paragraph.cc

@@ -479,8 +479,12 @@ void Paragraph::Layout(double width, bool force) {
       }
     }
 
-    // Exclude trailing whitespace from right-justified lines so the last
-    // visible character in the line will be flush with the right margin.
+    // TODO(garyq): Make GetRectsForRange return a zero-width box for the
+    // excluded whitespace such that the caret placement is correct regardless
+    // of the index being whitespace or not.
+
+    // Exclude trailing whitespace from right and center-justified lines so the
+    // last visible character in the line will be flush with the right margin.
     size_t line_end_index =
         (paragraph_style_.effective_align() == TextAlign::right ||
          paragraph_style_.effective_align() == TextAlign::center)