allow reverse order

src/textual/screen.py

@@ -1469,7 +1469,9 @@ def watch__select_end(self, select_end: tuple[Widget, Offset] | None) -> None:
 
         start_widget, start_offset = self._select_start
         end_widget, end_offset = select_end
-        self.selections = {start_widget: Selection(start_offset, end_offset)}
+        self.selections = {
+            start_widget: Selection.from_offsets(start_offset, end_offset)
+        }
 
     def dismiss(self, result: ScreenResultType | None = None) -> AwaitComplete:
         """Dismiss the screen, optionally with a result.

src/textual/selection.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from typing import NamedTuple
 
 from textual.geometry import Offset
@@ -11,6 +13,20 @@ class Selection(NamedTuple):
     end: Offset | None
     """Offset or None for `end`."""
 
+    @classmethod
+    def from_offsets(cls, offset1: Offset, offset2: Offset) -> Selection:
+        """Create selection from 2 offsets.
+
+        Args:
+            offset1: First offset.
+            offset2: Second offset.
+
+        Returns:
+            New Selection.
+        """
+        offsets = sorted([offset1, offset2], key=(lambda offset: (offset.y, offset.x)))
+        return cls(*offsets)
+
     def get_span(self, y: int) -> tuple[int, int] | None:
         """Get the selected span in a given line.