chore: Add changelog item

Author: gadenbuie

CHANGELOG.md

@@ -27,6 +27,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * `playwright.controller.InputActionButton` gains a `expect_icon()` method. As a result, the already existing `expect_label()` no longer includes the icon. (#2020)
 
+* `ui.sidebar()` gains a `fillable` argument to support vertical fill behavior in sidebars. (#2077)
+
 ### Changes
 
 * `express.ui.insert_accordion_panel()`'s function signature has changed to be more ergonomic. Now you can pass the `panel_title` and `panel_contents` directly instead of `ui.hold()`ing the `ui.accordion_panel()` context manager. (#2042)