fix(text area): fix mouse selection with tab characters (#5237)

Author: TomJGooding

Diff for: CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 - Fixed duplicated key displays in the help panel https://github.com/Textualize/textual/issues/5037
+- Fixed `TextArea` mouse selection with tab characters https://github.com/Textualize/textual/issues/5212
 
 ### Added
 

Diff for: src/textual/document/_wrapped_document.py

@@ -295,7 +295,9 @@ def offset_to_location(self, offset: Offset) -> Location:
         if not self._width:
             # No wrapping, so we directly map offset to location and clamp.
             line_index = min(y, len(self._wrap_offsets) - 1)
-            column_index = min(x, len(self.document.get_line(line_index)))
+            column_index = cell_width_to_column_index(
+                self.document.get_line(line_index), x, self._tab_width
+            )
             return line_index, column_index
 
         # Find the line corresponding to the given y offset in the wrapped document.

Diff for: tests/text_area/test_selection.py

@@ -334,3 +334,22 @@ def compose(self) -> ComposeResult:
 
         assert text_area.cursor_screen_offset == (5, 1)
         assert app.cursor_position == (5, 1)
+
+
+async def test_mouse_selection_with_tab_characters():
+    """Regression test for https://github.com/Textualize/textual/issues/5212"""
+
+    class TextAreaTabsApp(App):
+        def compose(self) -> ComposeResult:
+            yield TextArea(soft_wrap=False, text="\t\t")
+
+    app = TextAreaTabsApp()
+    async with app.run_test() as pilot:
+        text_area = pilot.app.query_one(TextArea)
+        expected_selection = Selection((0, 0), (0, 0))
+        assert text_area.selection == expected_selection
+
+        await pilot.mouse_down(text_area, offset=(2, 1))
+        await pilot.hover(text_area, offset=(3, 1))
+
+        assert text_area.selection == expected_selection