feat: cycle command completions backward with Shift+Tab

tests/unit/editor.rs

@@ -1080,8 +1080,10 @@ fn command_completion_cycles_core_alias_and_live_plugin_names_without_executing(
         assert_eq!(editor.context().text, document);
         assert!(!editor.should_quit());
     }
-    editor.handle(Input::Char('\t'));
-    editor.handle(Input::Char('\t'));
+    for expected in ["save_note", "save_and_quit", "save", "save_note"] {
+        assert_eq!(editor.handle(Input::BackTab), None);
+        assert_eq!(editor.command_line(), expected);
+    }
     let invocation = editor.handle(Input::Enter).unwrap();
     assert_eq!(invocation.plugin, "fixture");
     assert_eq!(invocation.command, "save_note");
@@ -1103,6 +1105,50 @@ fn command_completion_cycles_core_alias_and_live_plugin_names_without_executing(
     }
 }
 
+#[test]
+fn command_completion_reverse_starts_at_last_match_and_retains_prefix_on_direction_changes() {
+    let mut editor = editor();
+    type_text(&mut editor, ":tab.");
+    for (key, expected) in [
+        (Input::BackTab, "tab.previous"),
+        (Input::BackTab, "tab.next"),
+        (Input::Char('\t'), "tab.previous"),
+        (Input::Char('\t'), "tab.close"),
+        (Input::BackTab, "tab.previous"),
+    ] {
+        editor.handle(key);
+        assert_eq!(editor.command_line(), expected);
+        assert_eq!(editor.tab_count(), 1);
+    }
+    editor.handle(Input::Escape);
+    for draft in ["plugins", "unknown", "界", "edit file name"] {
+        type_text(&mut editor, ":");
+        type_text(&mut editor, draft);
+        for _ in 0..2 {
+            editor.handle(Input::BackTab);
+            assert_eq!(editor.command_line(), draft);
+        }
+        editor.handle(Input::Escape);
+    }
+    editor.bind("command", "shift+tab", None).unwrap();
+    editor
+        .bind(
+            "command",
+            "ctrl+b",
+            Some("core.command_complete_previous".into()),
+        )
+        .unwrap();
+    type_text(&mut editor, ":tab.");
+    editor.handle(Input::BackTab);
+    assert_eq!(editor.command_line(), "tab.");
+    editor.handle(Input::Ctrl('b'));
+    assert_eq!(editor.command_line(), "tab.previous");
+    editor.handle(Input::Char('\t'));
+    assert_eq!(editor.command_line(), "tab.close");
+    editor.handle(Input::Ctrl('b'));
+    assert_eq!(editor.command_line(), "tab.previous");
+}
+
 #[test]
 fn command_completion_restarts_after_edits_history_and_cancellation_and_ignores_arguments() {
     let mut editor = editor();