completions: Add subtle/eager behavior to Supermaven and Copilot (#35548)

This pull request introduces changes to improve the behavior and
consistency of multiple completion providers
(`CopilotCompletionProvider`, `SupermavenCompletionProvider`) and their
integration with UI elements like menus and inline completion buttons.
It now allows to see the prediction with the completion menu open whilst
pressing `opt` and also enables the subtle/eager setting that was
introduced with zeta.

Edit: I managed to get the preview working with correct icons!
<img width="909" height="232" alt="image"
src="https://github.com/user-attachments/assets/65800e67-4bc4-40f8-be78-806fcfe74ad9"
/>
<img width="1460" height="318" alt="CleanShot 2025-08-04 at 01 36 31@2x"
src="https://github.com/user-attachments/assets/15651405-720f-465f-a13c-c7470817810a"
/>

Correct icons are also displayed:
<img width="244" height="96" alt="image"
src="https://github.com/user-attachments/assets/0b8a687f-73e3-452d-aefb-784c52831b73"
/>


Edit2: I added some comments, would be very happy to receive feedback
(still learning rust)

Release Notes:

- Added Subtle and Eager edit prediction modes to Copilot and Supermaven

crates/supermaven/src/supermaven.rs

@@ -234,16 +234,14 @@ fn find_relevant_completion<'a>(
         }
 
         let original_cursor_offset = buffer.clip_offset(state.prefix_offset, text::Bias::Left);
-        let text_inserted_since_completion_request =
-            buffer.text_for_range(original_cursor_offset..current_cursor_offset);
-        let mut trimmed_completion = state_completion;
-        for chunk in text_inserted_since_completion_request {
-            if let Some(suffix) = trimmed_completion.strip_prefix(chunk) {
-                trimmed_completion = suffix;
-            } else {
-                continue 'completions;
-            }
-        }
+        let text_inserted_since_completion_request: String = buffer
+            .text_for_range(original_cursor_offset..current_cursor_offset)
+            .collect();
+        let trimmed_completion =
+            match state_completion.strip_prefix(&text_inserted_since_completion_request) {
+                Some(suffix) => suffix,
+                None => continue 'completions,
+            };
 
         if best_completion.map_or(false, |best| best.len() > trimmed_completion.len()) {
             continue;
@@ -439,3 +437,77 @@ pub struct SupermavenCompletion {
     pub id: SupermavenCompletionStateId,
     pub updates: watch::Receiver<()>,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use collections::BTreeMap;
+    use gpui::TestAppContext;
+    use language::Buffer;
+
+    #[gpui::test]
+    async fn test_find_relevant_completion_no_first_letter_skip(cx: &mut TestAppContext) {
+        let buffer = cx.new(|cx| Buffer::local("hello world", cx));
+        let buffer_snapshot = buffer.read_with(cx, |buffer, _| buffer.snapshot());
+
+        let mut states = BTreeMap::new();
+        let state_id = SupermavenCompletionStateId(1);
+        let (updates_tx, _) = watch::channel();
+
+        states.insert(
+            state_id,
+            SupermavenCompletionState {
+                buffer_id: buffer.entity_id(),
+                prefix_anchor: buffer_snapshot.anchor_before(0), // Start of buffer
+                prefix_offset: 0,
+                text: "hello".to_string(),
+                dedent: String::new(),
+                updates_tx,
+            },
+        );
+
+        let cursor_position = buffer_snapshot.anchor_after(1);
+
+        let result = find_relevant_completion(
+            &states,
+            buffer.entity_id(),
+            &buffer_snapshot,
+            cursor_position,
+        );
+
+        assert_eq!(result, Some("ello"));
+    }
+
+    #[gpui::test]
+    async fn test_find_relevant_completion_with_multiple_chars(cx: &mut TestAppContext) {
+        let buffer = cx.new(|cx| Buffer::local("hello world", cx));
+        let buffer_snapshot = buffer.read_with(cx, |buffer, _| buffer.snapshot());
+
+        let mut states = BTreeMap::new();
+        let state_id = SupermavenCompletionStateId(1);
+        let (updates_tx, _) = watch::channel();
+
+        states.insert(
+            state_id,
+            SupermavenCompletionState {
+                buffer_id: buffer.entity_id(),
+                prefix_anchor: buffer_snapshot.anchor_before(0), // Start of buffer
+                prefix_offset: 0,
+                text: "hello".to_string(),
+                dedent: String::new(),
+                updates_tx,
+            },
+        );
+
+        let cursor_position = buffer_snapshot.anchor_after(3);
+
+        let result = find_relevant_completion(
+            &states,
+            buffer.entity_id(),
+            &buffer_snapshot,
+            cursor_position,
+        );
+
+        assert_eq!(result, Some("lo"));
+    }
+}

crates/supermaven/src/supermaven_completion_provider.rs

@@ -108,6 +108,14 @@ impl EditPredictionProvider for SupermavenCompletionProvider {
     }
 
     fn show_completions_in_menu() -> bool {
+        true
+    }
+
+    fn show_tab_accept_marker() -> bool {
+        true
+    }
+
+    fn supports_jump_to_edit() -> bool {
         false
     }
 
@@ -116,7 +124,7 @@ impl EditPredictionProvider for SupermavenCompletionProvider {
     }
 
     fn is_refreshing(&self) -> bool {
-        self.pending_refresh.is_some()
+        self.pending_refresh.is_some() && self.completion_id.is_none()
     }
 
     fn refresh(
@@ -197,6 +205,7 @@ impl EditPredictionProvider for SupermavenCompletionProvider {
             let mut point = cursor_position.to_point(&snapshot);
             point.column = snapshot.line_len(point.row);
             let range = cursor_position..snapshot.anchor_after(point);
+
             Some(completion_from_diff(
                 snapshot,
                 completion_text,