Provide an optional view reference in CompletionContext

FEATURE: `CompletionContext` objects now have a `view` property that holds
the editor view when the query context has a view available.

src/completion.ts

@@ -96,7 +96,13 @@ export class CompletionContext {
     /// implicitly by typing. The usual way to respond to this is to
     /// only return completions when either there is part of a
     /// completable entity before the cursor, or `explicit` is true.
-    readonly explicit: boolean
+    readonly explicit: boolean,
+    /// The editor view. May be undefined if the context was created
+    /// in a situation where there is no such view available, such as
+    /// in synchronous updates via
+    /// [`CompletionResult.update`](#autocomplete.CompletionResult.update)
+    /// or when called by test code.
+    readonly view?: EditorView
   ) {}
 
   /// Get the extent, content, and (if there is a token) type of the

src/view.ts

@@ -130,7 +130,7 @@ export const completionPlugin = ViewPlugin.fromClass(class implements PluginValu
 
   startQuery(active: ActiveSource) {
     let {state} = this.view, pos = cur(state)
-    let context = new CompletionContext(state, pos, active.explicitPos == pos)
+    let context = new CompletionContext(state, pos, active.explicitPos == pos, this.view)
     let pending = new RunningQuery(active, context)
     this.running.push(pending)
     Promise.resolve(active.source(context)).then(result => {