Don’t crash when line can be split before trailing spaces

We normally don’t break lines before spaces, but some characters (such as line
separators) can force line breaks anywhere.

Fix #1852.

tests/test_text.py

@@ -1025,6 +1025,19 @@ def test_overflow_wrap_trailing_space(wrap, text, body_width, expected_width):
     assert td.width == expected_width
 
 
+def test_line_break_before_trailing_space():
+    # Test regression: https://github.com/Kozea/WeasyPrint/issues/1852
+    page, = render_pages('''
+        <p style="display: inline-block">test\u2028 </p>a
+        <p style="display: inline-block">test\u2028</p>a
+    ''')
+    html, = page.children
+    body, = html.children
+    line, = body.children
+    p1, space1, p2, space2 = line.children
+    assert p1.width == p2.width
+
+
 def white_space_lines(width, space):
     page, = render_pages('''
       <style>

weasyprint/layout/preferred.py

@@ -744,9 +744,12 @@ def trailing_whitespace_size(context, box):
         stripped_box = box.copy_with_text(stripped_text)
         stripped_box, resume, _ = split_text_box(
             context, stripped_box, None, old_resume)
-        assert stripped_box is not None
-        assert resume is None
-        return old_box.width - stripped_box.width
+        if stripped_box is None:
+            # old_box split just before the trailing spaces
+            return old_box.width
+        else:
+            assert resume is None
+            return old_box.width - stripped_box.width
     else:
         _, _, _, width, _, _ = split_first_line(
             box.text, box.style, context, None, box.justification_spacing)