fix(masked input): highlight selected text

Fixes #5495.

Author: TomJGooding

src/textual/widgets/_masked_input.py

@@ -569,12 +569,19 @@ def render_line(self, y: int) -> Strip:
             if char == " ":
                 result.stylize(style, index, index + 1)
 
-        if self._cursor_visible and self.has_focus:
-            if self._cursor_at_end:
-                result.pad_right(1)
-            cursor_style = self.get_component_rich_style("input--cursor")
-            cursor = self.cursor_position
-            result.stylize(cursor_style, cursor, cursor + 1)
+        if self.has_focus:
+            if not self.selection.is_empty:
+                start, end = self.selection
+                start, end = sorted((start, end))
+                selection_style = self.get_component_rich_style("input--selection")
+                result.stylize_before(selection_style, start, end)
+
+            if self._cursor_visible:
+                cursor_style = self.get_component_rich_style("input--cursor")
+                cursor = self.cursor_position
+                if self._cursor_at_end:
+                    result.pad_right(1)
+                result.stylize(cursor_style, cursor, cursor + 1)
 
         segments = list(result.render(self.app.console))
         line_length = Segment.get_line_length(segments)