Add single width left margin for completion popup (helix-editor#2728)

* Add single width left margin for completion popup

* Clear with ui.menu style before rendering menu

When rendering a completion popup, the popup component will clear
the area with ui.popup and then the menu component would draw over
it using a table component. We remove the left edge of the area
before passing it to the table component (so that it will be left
as padding), and the table component uses ui.menu as the style.
If ui.menu and ui.popup are different the left edge of the popup
will look different from the rest of the popup. We avoid this by
clearing the whole area with ui.menu in Menu::render

helix-term/src/ui/menu.rs

@@ -48,6 +48,8 @@ pub struct Menu<T: Item> {
 }
 
 impl<T: Item> Menu<T> {
+    const LEFT_PADDING: usize = 1;
+
     // TODO: it's like a slimmed down picker, share code? (picker = menu + prompt with different
     // rendering)
     pub fn new(
@@ -150,6 +152,7 @@ impl<T: Item> Menu<T> {
             len += 1; // +1: reserve some space for scrollbar
         }
 
+        len += Self::LEFT_PADDING;
         let width = len.min(viewport.0 as usize);
 
         self.widths = max_lens
@@ -271,6 +274,7 @@ impl<T: Item + 'static> Component for Menu<T> {
             .try_get("ui.menu")
             .unwrap_or_else(|| theme.get("ui.text"));
         let selected = theme.get("ui.menu.selected");
+        surface.clear_with(area, style);
 
         let scroll = self.scroll;
 
@@ -306,14 +310,20 @@ impl<T: Item + 'static> Component for Menu<T> {
         use tui::widgets::TableState;
 
         table.render_table(
-            area,
+            area.clip_left(Self::LEFT_PADDING as u16),
             surface,
             &mut TableState {
                 offset: scroll,
                 selected: self.cursor,
             },
         );
 
+        if let Some(cursor) = self.cursor {
+            let offset_from_top = cursor - scroll;
+            let cell = &mut surface[(area.x, area.y + offset_from_top as u16)];
+            cell.set_style(selected);
+        }
+
         let fits = len <= win_height;
 
         for (i, _) in (scroll..(scroll + win_height).min(len)).enumerate() {