Make completions selector continue to show docs aside if ever shown (#21704)

In #21286, documentation fetch was made more efficient by only
fetching the current completion. This has a side effect of causing the
aside to disappear and reappear when navigating the list. This is
particularly jarring when there isn't enough space for the aside,
causing the completions list to jump to the left.

The solution here is to continue to show the aside even if the current
selection does not yet have docs fetched.

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -141,7 +141,7 @@ use snippet::Snippet;
 use std::{
     any::TypeId,
     borrow::Cow,
-    cell::RefCell,
+    cell::{Cell, RefCell},
     cmp::{self, Ordering, Reverse},
     mem,
     num::NonZeroU32,
@@ -1008,6 +1008,7 @@ struct CompletionsMenu {
     selected_item: usize,
     scroll_handle: UniformListScrollHandle,
     selected_completion_resolve_debounce: Option<Arc<Mutex<DebouncedDelay>>>,
+    aside_was_displayed: Cell<bool>,
 }
 
 impl CompletionsMenu {
@@ -1040,6 +1041,7 @@ impl CompletionsMenu {
             selected_item: 0,
             scroll_handle: UniformListScrollHandle::new(),
             selected_completion_resolve_debounce: Some(Arc::new(Mutex::new(DebouncedDelay::new()))),
+            aside_was_displayed: Cell::new(false),
         }
     }
 
@@ -1093,6 +1095,7 @@ impl CompletionsMenu {
             selected_item: 0,
             scroll_handle: UniformListScrollHandle::new(),
             selected_completion_resolve_debounce: Some(Arc::new(Mutex::new(DebouncedDelay::new()))),
+            aside_was_displayed: Cell::new(false),
         }
     }
 
@@ -1231,7 +1234,7 @@ impl CompletionsMenu {
 
         let multiline_docs = if show_completion_documentation {
             let mat = &self.matches[selected_item];
-            let multiline_docs = match &self.completions.read()[mat.candidate_id].documentation {
+            match &self.completions.read()[mat.candidate_id].documentation {
                 Some(Documentation::MultiLinePlainText(text)) => {
                     Some(div().child(SharedString::from(text.clone())))
                 }
@@ -1244,24 +1247,37 @@ impl CompletionsMenu {
                         cx,
                     )))
                 }
+                Some(Documentation::Undocumented) if self.aside_was_displayed.get() => {
+                    Some(div().child("No documentation"))
+                }
                 _ => None,
-            };
-            multiline_docs.map(|div| {
-                div.id("multiline_docs")
-                    .max_h(max_height)
-                    .flex_1()
-                    .px_1p5()
-                    .py_1()
-                    .min_w(px(260.))
-                    .max_w(px(640.))
-                    .w(px(500.))
-                    .overflow_y_scroll()
-                    .occlude()
-            })
+            }
         } else {
             None
         };
 
+        let aside_contents = if let Some(multiline_docs) = multiline_docs {
+            Some(multiline_docs)
+        } else if self.aside_was_displayed.get() {
+            Some(div().child("Fetching documentation..."))
+        } else {
+            None
+        };
+        self.aside_was_displayed.set(aside_contents.is_some());
+
+        let aside_contents = aside_contents.map(|div| {
+            div.id("multiline_docs")
+                .max_h(max_height)
+                .flex_1()
+                .px_1p5()
+                .py_1()
+                .min_w(px(260.))
+                .max_w(px(640.))
+                .w(px(500.))
+                .overflow_y_scroll()
+                .occlude()
+        });
+
         let list = uniform_list(
             cx.view().clone(),
             "completions",
@@ -1357,8 +1373,8 @@ impl CompletionsMenu {
 
         Popover::new()
             .child(list)
-            .when_some(multiline_docs, |popover, multiline_docs| {
-                popover.aside(multiline_docs)
+            .when_some(aside_contents, |popover, aside_contents| {
+                popover.aside(aside_contents)
             })
             .into_any_element()
     }