Fix highlighting edges and unify highlighting code

tests/features/syntax_highlight_test.py

@@ -78,3 +78,22 @@ def test_syntax_highlighting_does_not_highlight_arrows(run, tmpdir):
     with run(str(f), term='screen-256color', width=20) as h, and_exit(h):
         h.await_text('loooo')
         h.assert_screen_attr_equals(2, [(243, 40, 0)] * 19 + [(236, 40, 0)])
+
+        h.press('Down')
+        h.press('^E')
+        h.await_text_missing('loooo')
+        expected = [(236, 40, 0)] + [(243, 40, 0)] * 15 + [(236, 40, 0)] * 4
+        h.assert_screen_attr_equals(2, expected)
+
+
+def test_syntax_highlighting_off_screen_does_not_crash(run, tmpdir):
+    f = tmpdir.join('f.demo')
+    f.write(f'"""a"""{"x" * 40}"""b"""')
+
+    with run(str(f), term='screen-256color', width=20) as h, and_exit(h):
+        h.await_text('"""a"""')
+        h.assert_screen_attr_equals(1, [(17, 40, 0)] * 7 + [(236, 40, 0)] * 13)
+        h.press('^E')
+        h.await_text('"""b"""')
+        expected = [(236, 40, 0)] * 11 + [(17, 40, 0)] * 7 + [(236, 40, 0)] * 2
+        h.assert_screen_attr_equals(1, expected)

tests/features/undo_redo_test.py

@@ -127,3 +127,16 @@ def test_undo_redo_causes_scroll(run):
         h.await_cursor_position(x=0, y=1)
         h.press('M-U')
         h.await_cursor_position(x=0, y=4)
+
+
+def test_undo_redo_clears_selection(run, ten_lines):
+    # maintaining the selection across undo/redo is both difficult and not all
+    # that useful.  prior to this it was buggy anyway (a negative selection
+    # indented and then undone would highlight out of bounds)
+    with run(str(ten_lines), width=20) as h, and_exit(h):
+        h.press('S-Down')
+        h.press('Tab')
+        h.await_cursor_position(x=4, y=2)
+        h.press('M-u')
+        h.await_cursor_position(x=0, y=2)
+        h.assert_screen_attr_equals(1, [(-1, -1, 0)] * 20)