Show the primary selection index on statusline

Have the status line show the number of the current selection out of
the total in the file.

Right now, the `# sels` shows how many selections you have in the current
buffer, but this shows the (1-based) index at which which one your
primary cursor is on in the current buffer, i.e. `3/6 sels`, then if you
press `)`, it would say `4/6` sels etc.

This is very helpful when multiple selections are spread out beyond
a single view to orient oneself with the current selections.

book/src/editor.md

@@ -138,7 +138,7 @@ The following statusline elements can be configured:
 | `file-type` | The type of the opened file |
 | `diagnostics` | The number of warnings and/or errors |
 | `workspace-diagnostics` | The number of warnings and/or errors on workspace |
-| `selections` | The number of active selections |
+| `selections` | The primary selection index out of the number of active selections |
 | `primary-selection-length` | The number of characters currently in primary selection |
 | `position` | The cursor position |
 | `position-percentage` | The cursor position as a percentage of the total number of lines |

helix-term/src/ui/statusline.rs

@@ -347,10 +347,16 @@ fn render_selections<F>(context: &mut RenderContext, write: F)
 where
     F: Fn(&mut RenderContext, String, Option<Style>) + Copy,
 {
-    let count = context.doc.selection(context.view.id).len();
+    let selection = context.doc.selection(context.view.id);
+    let count = selection.len();
     write(
         context,
-        format!(" {} sel{} ", count, if count == 1 { "" } else { "s" }),
+        format!(
+            "{}/{} sel{} ",
+            selection.primary_index() + 1,
+            count,
+            if count == 1 { "" } else { "s" }
+        ),
         None,
     );
 }