Add always_update as a parameter for a var reactive

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - The DataTable cursor is now scrolled into view when the cursor coordinate is changed programmatically https://github.com/Textualize/textual/issues/2459
+- Added `always_update` as an optional argument for `reactive.var`
 
 ## [0.23.0] - 2023-05-03
 

src/textual/widgets/_tree.py

@@ -429,7 +429,7 @@ class Tree(Generic[TreeDataType], ScrollView, can_focus=True):
     """Show the root of the tree."""
     hover_line = var(-1)
     """The line number under the mouse pointer, or -1 if not under the mouse pointer."""
-    cursor_line = var(-1)
+    cursor_line = var(-1, always_update=True)
     """The line with the cursor, or -1 if no cursor."""
     show_guides = reactive(True)
     """Enable display of tree guide lines."""