Fix a TextArea crash (#4126)

* Fix crash with backwards selection where content is replaced with fewer lines of text

* Ensure correct cursor positioning after paste

* Improving tests

* Update CHANGELOG

* Add missing docstrings

Author: darrenburns

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+
+- Fixed a crash in the TextArea when performing a backward replace https://github.com/Textualize/textual/pull/4126
+- Fixed selection not updating correctly when pasting while there's a non-zero selection https://github.com/Textualize/textual/pull/4126
+- Breaking change: `TextArea` will not use `Escape` to shift focus if the `tab_behaviour` is the default https://github.com/Textualize/textual/issues/4110
+
 ### Added
 
 - Added DOMQuery.set https://github.com/Textualize/textual/pull/4075
@@ -15,10 +21,6 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added DOMNode.action_toggle https://github.com/Textualize/textual/pull/4075
 - Added Worker.cancelled_event https://github.com/Textualize/textual/pull/4075
 
-### Fixed
-
-- Breaking change: `TextArea` will not use `Escape` to shift focus if the `tab_behaviour` is the default https://github.com/Textualize/textual/issues/4110
-
 ## [0.48.2] - 2024-02-02
 
 ### Fixed

src/textual/widgets/_text_area.py

@@ -958,7 +958,6 @@ def render_line(self, widget_y: int) -> Strip:
         # Get the line from the Document.
         line_string = document.get_line(line_index)
         line = Text(line_string, end="")
-
         line_character_count = len(line)
         line.tab_size = self.indent_width
         line.set_length(line_character_count + 1)  # space at end for cursor
@@ -1193,8 +1192,8 @@ def edit(self, edit: Edit) -> EditResult:
             self.wrapped_document.wrap(self.wrap_width, self.indent_width)
         else:
             self.wrapped_document.wrap_range(
-                edit.from_location,
-                edit.to_location,
+                edit.top,
+                edit.bottom,
                 result.end_location,
             )
 
@@ -1341,7 +1340,8 @@ async def _on_mouse_up(self, event: events.MouseUp) -> None:
 
     async def _on_paste(self, event: events.Paste) -> None:
         """When a paste occurs, insert the text from the paste event into the document."""
-        self.replace(event.text, *self.selection)
+        result = self.replace(event.text, *self.selection)
+        self.move_cursor(result.end_location)
 
     def cell_width_to_column_index(self, cell_width: int, row_index: int) -> int:
         """Return the column that the cell width corresponds to on the given row.
@@ -1814,8 +1814,7 @@ def delete(
         Returns:
             An `EditResult` containing information about the edit.
         """
-        top, bottom = sorted((start, end))
-        return self.edit(Edit("", top, bottom, maintain_selection_offset))
+        return self.edit(Edit("", start, end, maintain_selection_offset))
 
     def replace(
         self,
@@ -1989,14 +1988,13 @@ def do(self, text_area: TextArea) -> EditResult:
         # position in the document even if an insert happens before
         # their cursor position.
 
-        edit_top, edit_bottom = sorted((edit_from, edit_to))
-        edit_bottom_row, edit_bottom_column = edit_bottom
+        edit_bottom_row, edit_bottom_column = self.bottom
 
         selection_start, selection_end = text_area.selection
         selection_start_row, selection_start_column = selection_start
         selection_end_row, selection_end_column = selection_end
 
-        replace_result = text_area.document.replace_range(edit_from, edit_to, text)
+        replace_result = text_area.document.replace_range(self.top, self.bottom, text)
 
         new_edit_to_row, new_edit_to_column = replace_result.end_location
 
@@ -2051,6 +2049,16 @@ def after(self, text_area: TextArea) -> None:
             text_area.selection = self._updated_selection
         text_area.record_cursor_width()
 
+    @property
+    def top(self) -> Location:
+        """The Location impacted by this edit that is nearest the start of the document."""
+        return min([self.from_location, self.to_location])
+
+    @property
+    def bottom(self) -> Location:
+        """The Location impacted by this edit that is nearest the end of the document."""
+        return max([self.from_location, self.to_location])
+
 
 @runtime_checkable
 class Undoable(Protocol):

tests/text_area/test_edit_via_bindings.py

@@ -6,9 +6,11 @@
 
 Note that more extensive testing for editing is done at the Document level.
 """
+
 import pytest
 
 from textual.app import App, ComposeResult
+from textual.events import Paste
 from textual.widgets import TextArea
 from textual.widgets.text_area import Selection
 
@@ -416,3 +418,54 @@ async def test_delete_word_right_at_end_of_line():
 
         assert text_area.text == "0123456789"
         assert text_area.selection == Selection.cursor((0, 5))
+
+
+@pytest.mark.parametrize(
+    "selection",
+    [
+        Selection(start=(1, 0), end=(3, 0)),
+        Selection(start=(3, 0), end=(1, 0)),
+    ],
+)
+async def test_replace_lines_with_fewer_lines(selection):
+    app = TextAreaApp()
+    async with app.run_test() as pilot:
+        text_area = app.query_one(TextArea)
+        text_area.text = SIMPLE_TEXT
+        text_area.selection = selection
+
+        await pilot.press("a")
+
+        expected_text = """\
+ABCDE
+aPQRST
+UVWXY
+Z"""
+        assert text_area.text == expected_text
+        assert text_area.selection == Selection.cursor((1, 1))
+
+
+@pytest.mark.parametrize(
+    "selection",
+    [
+        Selection(start=(1, 0), end=(3, 0)),
+        Selection(start=(3, 0), end=(1, 0)),
+    ],
+)
+async def test_paste(selection):
+    app = TextAreaApp()
+    async with app.run_test() as pilot:
+        text_area = app.query_one(TextArea)
+        text_area.text = SIMPLE_TEXT
+        text_area.selection = selection
+
+        app.post_message(Paste("a"))
+        await pilot.pause()
+
+        expected_text = """\
+ABCDE
+aPQRST
+UVWXY
+Z"""
+        assert text_area.text == expected_text
+        assert text_area.selection == Selection.cursor((1, 1))