Fix marking of text not working on lines that contain zero cells

Zero cells are passed to the regex engine as spaces, so they must
increment the match_pos counter. Fixes #3403

docs/changelog.rst

@@ -105,6 +105,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Graphics protocol: Add a control to allow clients to specify that the cursor
   should not move when displaying an image (:iss:`3411`)
 
+- Fix marking of text not working on lines that contain zero cells
+  (:iss:`3403`)
+
 
 0.19.3 [2020-12-19]
 -------------------

kitty/line.c

@@ -736,8 +736,8 @@ apply_mark(Line *line, const attrs_type mark, index_type *cell_pos, unsigned int
 #define MARK { line->gpu_cells[x].attrs &= ATTRS_MASK_WITHOUT_MARK; line->gpu_cells[x].attrs |= mark; }
     index_type x = *cell_pos;
     MARK;
+    (*match_pos)++;
     if (line->cpu_cells[x].ch) {
-        (*match_pos)++;
         if (line->cpu_cells[x].ch == '\t') {
             unsigned num_cells_to_skip_for_tab = line->cpu_cells[x].cc_idx[0];
             while (num_cells_to_skip_for_tab && x + 1 < line->xnum && line->cpu_cells[x+1].ch == ' ') {

kitty_tests/screen.py

@@ -647,6 +647,13 @@ class TestScreen(BaseTest):
         self.ae(s.marked_cells(), cells(8))
         s.set_marker(marker_from_regex('\t', 3))
         self.ae(s.marked_cells(), cells(*range(8)))
+        s = self.create_screen()
+        s.cursor.x = 2
+        s.draw('x')
+        s.cursor.x += 1
+        s.draw('x')
+        s.set_marker(marker_from_function(mark_x))
+        self.ae(s.marked_cells(), [(2, 0, 1), (4, 0, 2)])
 
     def test_hyperlinks(self):
         s = self.create_screen()