ignore Enter keypress when menu has no selection (#1704)

* ignore Enter keypress when menu has no selection

supersedes #1622

Builds on the work in #1285. I want to allow Enter to create a newline
when there is no selection in the autocomplete menu.

This occurs somewhat often when using LSP autocomplete in Elixir which
uses `do/end` blocks (and I set the autocomplete menu delay to 0 which
exacerbates the problem):

```elixir
defmodule MyModule do
  def do_foo(x) do
    x
  end
  def other_function(y) do|
end
```

Here the cursor is `|` in insert mode. The LSP suggests `do_foo` but I
want to create a newline. Hitting Enter currently closes the menu,
so I end up having to hit Enter twice when the module contains any
local with a `do` prefix, which can be inconsistent. With this change,
we ignore the Enter keypress to end up creating the newline in this case.

* pop compositor layer when ignoring Enter keypress

* move closing function out of consumed event result closure

* explicitly label close_fn as an 'Option<Callback>'

helix-term/src/ui/menu.rs

@@ -1,5 +1,5 @@
 use crate::{
-    compositor::{Component, Compositor, Context, EventResult},
+    compositor::{Callback, Component, Compositor, Context, EventResult},
     ctrl, key, shift,
 };
 use crossterm::event::Event;
@@ -205,16 +205,16 @@ impl<T: Item + 'static> Component for Menu<T> {
             _ => return EventResult::Ignored(None),
         };
 
-        let close_fn = EventResult::Consumed(Some(Box::new(|compositor: &mut Compositor, _| {
+        let close_fn: Option<Callback> = Some(Box::new(|compositor: &mut Compositor, _| {
             // remove the layer
             compositor.pop();
-        })));
+        }));
 
         match event.into() {
             // esc or ctrl-c aborts the completion and closes the menu
             key!(Esc) | ctrl!('c') => {
                 (self.callback_fn)(cx.editor, self.selection(), MenuEvent::Abort);
-                return close_fn;
+                return EventResult::Consumed(close_fn);
             }
             // arrow up/ctrl-p/shift-tab prev completion choice (including updating the doc)
             shift!(Tab) | key!(Up) | ctrl!('p') | ctrl!('k') => {
@@ -231,8 +231,10 @@ impl<T: Item + 'static> Component for Menu<T> {
             key!(Enter) => {
                 if let Some(selection) = self.selection() {
                     (self.callback_fn)(cx.editor, Some(selection), MenuEvent::Validate);
+                    return EventResult::Consumed(close_fn);
+                } else {
+                    return EventResult::Ignored(close_fn);
                 }
-                return close_fn;
             }
             // KeyEvent {
             //     code: KeyCode::Char(c),