From b148ab2aa9012c60632a6e0aaf2db5465b7a9b34 Mon Sep 17 00:00:00 2001
From: TomJGooding <101601846+TomJGooding@users.noreply.github.com>
Date: Fri, 23 May 2025 14:52:31 +0100
Subject: [PATCH] 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
---
 CHANGELOG.md                         |  1 +
 src/textual/widgets/_input.py        |  4 ++++
 tests/input/test_input_properties.py | 13 +++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 072f92175..86d28a677 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/src/textual/widgets/_input.py b/src/textual/widgets/_input.py
index e70c69407..a9e80c696 100644
--- a/src/textual/widgets/_input.py
+++ b/src/textual/widgets/_input.py
@@ -536,6 +536,10 @@ class Input(ScrollView):
         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."""
diff --git a/tests/input/test_input_properties.py b/tests/input/test_input_properties.py
index ef518ba69..779254932 100644
--- a/tests/input/test_input_properties.py
+++ b/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))