allow optionally highlighting the area past the end of the file

this makes it easier to see when the file is actually ended, rather
than just having a bunch of whitespace near the bottom of the terminal

book/src/themes.md

@@ -268,6 +268,7 @@ These scopes are used for theming the editor interface:
 | ---                               | ---                                                                                            |
 | `ui.background`                   |                                                                                                |
 | `ui.background.separator`         | Picker separator below input line                                                              |
+| `ui.background.eof`               | Portion of the editor window past the end of the buffer contents                               |
 | `ui.cursor`                       |                                                                                                |
 | `ui.cursor.normal`                |                                                                                                |
 | `ui.cursor.insert`                |                                                                                                |

helix-term/src/ui/document.rs

@@ -347,6 +347,11 @@ pub fn render_text<'t>(
         );
     }
 
+    renderer.surface.set_style(
+        renderer.viewport.clip_top(last_line_pos.visual_line),
+        theme.get("ui.background.eof"),
+    );
+
     renderer.draw_indent_guides(last_line_indent_level, last_line_pos.visual_line);
     for line_decoration in &mut *line_decorations {
         line_decoration.render_foreground(renderer, last_line_pos, char_pos);