Fix uncut selection at end of file

thanks @YouTwitFace for the report!

babi/file.py

@@ -623,6 +623,7 @@ class File:
         self.buf.up(margin)
         self.buf.x = len(self.buf[self.buf.y])
         self.buf[self.buf.y] += self.buf.pop(self.buf.y + 1)
+        self.buf.restore_eof_invariant()
 
     def _sort(self, margin: Margin, s_y: int, e_y: int) -> None:
         # self.buf intentionally does not support slicing so we use islice

babi/hl/trailing_whitespace.py

@@ -35,8 +35,8 @@ class TrailingWhitespace:
             self.regions[idx] = self._trailing_ws(lines[idx])
 
     def _del_cb(self, lines: Buf, idx: int, victim: str) -> None:
-        assert idx < len(self.regions)  # currently all `del` happen on screen
+        if idx < len(self.regions):
-        del self.regions[idx]
+            del self.regions[idx]
 
     def _ins_cb(self, lines: Buf, idx: int) -> None:
         if idx < len(self.regions):

tests/features/cut_uncut_test.py

@@ -132,3 +132,21 @@ def test_selection_cut_uncut_selection_offscreen_x(run):
         h.await_text_missing('hello')
         h.press('^K')
         h.await_text('hello\n')
+
+
+def test_selection_cut_uncut_at_end_of_file(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('S-Down')
+        h.press('S-Right')
+        h.press('^K')
+        h.await_text_missing('line_0')
+        h.await_text_missing('line_1')
+        h.await_text('ine_1')
+
+        h.press('^End')
+        h.press('^U')
+        h.await_text('line_0\nl\n')
+        h.await_cursor_position(x=1, y=11)
+
+        h.press('Down')
+        h.await_cursor_position(x=0, y=12)