handle tabs correctly when highlighting text

With the old solution I replaced tabs with four spaces and then did
some calculations to get the correct byte positions for a coordinate.
With the new solution (using FontMetric.horizontalAdvance) this is not
necessary anymore and we can use tha native tab.
But I cannot change the tab width. It is always 8 characters.
You can set tabStopDistance on horizontalAdvance(), but not on
elided_text() and I did not find a way to use it while rendering text.

src/ui/bigtext/line.py

@@ -15,7 +15,7 @@ class Line:
         self._cache_char_to_column()
 
     def get_width_in_px(self, font_metric: QFontMetrics):
-        return font_metric.horizontalAdvance(self._line)
+        return font_metric.horizontalAdvance(self.line_prepared_for_display())
 
     def byte_offset(self) -> int:
         return self._byte_offset
@@ -43,7 +43,8 @@ class Line:
         return len(prefix_chars)
 
     def line_prepared_for_display(self) -> str:
-        line = self._line_tabs_replaced()
+        # line = self._line_tabs_replaced()
+        line = self._line
         line = self._replace_control_chars_with_pictures(line)
         return line
 
@@ -108,7 +109,7 @@ class Line:
             if not result in self._column_to_char_cache:
                 self._column_to_char_cache[result] = i
             current_char = self._line[i]
-            if current_char == "\t":
+            if False and current_char == "\t":
                 result = result + constants.tab_width - result % constants.tab_width
             else:
                 result = result + 1

src/ui/bigtext/testline.py

@@ -78,20 +78,6 @@ class MyTestCase(unittest.TestCase):
         self.assertEqual(2, line.char_to_column(4))  # z̈
         self.assertEqual(2, line.char_to_column(5))  # z̈
 
-    def test_line_tabs_replaced(self):
-        byte_offset = 123
-        text = "\ta\tb"  # will be rendered as: ....abc  where . represents a whitespace column
-        expected = "    a   b"
-        line = Line(byte_offset=byte_offset, byte_end=byte_offset + len(text.encode("utf8")), line=text)
-        self.assertEqual(expected, line.line_prepared_for_display())
-
-    def test_line_tabs_replaced_performance(self):
-        byte_offset = 123
-        text = "a\t" * 10000
-        expected = "a   " * 10000
-        line = Line(byte_offset=byte_offset, byte_end=byte_offset + len(text.encode("utf8")), line=text)
-        self.assertEqual(expected, line.line_prepared_for_display())
-
     def test_byte_index_to_char_index(self):
         byte_offset = 123
         text = "x\u0308y\u0308z\u0308\t\u0308a"