fix(input): fix invalid cursor position after updating value

Validate the `Input` selection when the value changes, to ensure it
accounts for the length of the new value.

Fixes #5811

Author: TomJGooding

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fixed `VERTICAL_BREAKPOINTS` doesn't work https://github.com/Textualize/textual/pull/5785
 - Fixed `Button` allowing text selection https://github.com/Textualize/textual/pull/5770
+- Fixed `Input` invalid cursor position after updating the value https://github.com/Textualize/textual/issues/5811
 
 ## [3.2.0] - 2025-05-02
 

src/textual/widgets/_input.py

@@ -536,6 +536,10 @@ def _watch_value(self, value: str) -> None:
         if self._initial_value:
             self.cursor_position = len(self.value)
             self._initial_value = False
+        else:
+            # Force a re-validation of the selection to ensure it accounts for
+            # the length of the new value
+            self.selection = self.selection
 
     def _watch_valid_empty(self) -> None:
         """Repeat validation when valid_empty changes."""

tests/input/test_input_properties.py

@@ -93,3 +93,16 @@ async def test_input_selection_deleted_programmatically():
         input_widget.selection = Selection(4, 0)
         input_widget.delete_selection()
         assert input_widget.value == "o, world!"
+
+
+async def test_input_selection_is_valid_after_updating_value():
+    """Regression test for https://github.com/Textualize/textual/issues/5811"""
+    app = InputApp()
+    async with app.run_test() as pilot:
+        input_widget = pilot.app.query_one(Input)
+        # Sanity check (by default focusing the input selects all text)
+        assert input_widget.selection == (0, len(input_widget.value))
+
+        input_widget.value = "foo"
+
+        assert input_widget.selection == (0, len(input_widget.value))