Ensure hover effect doesnt linger when mouse leaves Tree (#4766)

* Ensure hover effect doesnt linger when mouse leaves Tree

* Update CHANGELOG

Author: darrenburns

CHANGELOG.md

@@ -35,6 +35,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - `TreeNodeSelected` messages are now posted before `TreeNodeExpanded` messages
 when an expandable node is selected https://github.com/Textualize/textual/pull/4753
 - `Markdown.LinkClicked.href` is now automatically unquoted https://github.com/Textualize/textual/pull/4749
+- The mouse cursor hover effect of `Tree` and `DirectoryTree` will no longer linger after the mouse leaves the widget https://github.com/Textualize/textual/pull/4766
 
 
 ## [0.72.0] - 2024-07-09

src/textual/widgets/_tree.py

@@ -839,6 +839,10 @@ def _on_mouse_move(self, event: events.MouseMove) -> None:
         else:
             self.hover_line = -1
 
+    def _on_leave(self, _: events.Leave) -> None:
+        # Ensure the hover effect doesn't linger after the mouse leaves.
+        self.hover_line = -1
+
     def _new_id(self) -> NodeID:
         """Create a new node ID.