Fix a linebox breaking corner case

Fix #660.

As said in the (very helpful) comment at the beginning of the modified block,
we have to add "child_resume_at" and "initial_skip_stack[1]" skip stacks, but
the old algorithm was obviously wrong when the stacks had more than one level
of depth.

The fix is shorter, easier to read and right (until next bug). A regression
test has been added too.

weasyprint/layout/inlines.py

@@ -856,22 +856,20 @@ def split_inline_box(context, box, position_x, max_x, skip_stack,
                                 # We have to do:
                                 # child_resume_at += initial_skip_stack[1]
                                 # but adding skip stacks is a bit complicated
-                                current_skip, grandchild_skip = (
-                                    initial_skip_stack[1])
-                                current_resume_at, grandchild_resume_at = (
-                                    child_resume_at)
-                                if grandchild_skip is None:
-                                    grandchild_resume_at = child_resume_at[1]
-                                elif grandchild_resume_at is None:
-                                    grandchild_resume_at = grandchild_skip
-                                else:
-                                    grandchild_resume_at = (
-                                        grandchild_skip[0] +
-                                        grandchild_resume_at[0],
-                                        None)
+                                current_skip_stack = initial_skip_stack[1]
+                                current_resume_at = child_resume_at
+                                stack = []
+                                while current_skip_stack and current_resume_at:
+                                    skip_stack, current_skip_stack = (
+                                        current_skip_stack)
+                                    resume_at, current_resume_at = (
+                                        current_resume_at)
+                                    stack.append(skip_stack + resume_at)
                                 child_resume_at = (
-                                    current_resume_at + current_skip,
-                                    grandchild_resume_at)
+                                    current_skip_stack or current_resume_at)
+                                while stack:
+                                    child_resume_at = (
+                                        stack.pop(), child_resume_at)
 
                             resume_at = (child_index, child_resume_at)
                             break

weasyprint/tests/test_layout/test_inline.py

@@ -244,6 +244,23 @@ def test_breaking_linebox_regression_6():
     assert line2.children[0].children[0].text == '/ccc'
 
 
+@assert_no_logs
+def test_breaking_linebox_regression_7():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/660
+    page, = parse(
+      '<style>@font-face { src: url(AHEM____.TTF); font-family: ahem }</style>'
+      '<div style="width: 3.5em; font-family: ahem">'
+      '<span><span>abc d e</span></span><span>f')
+    html, = page.children
+    body, = html.children
+    div, = body.children
+    line1, line2, line3 = div.children
+    assert line1.children[0].children[0].children[0].text == 'abc'
+    assert line2.children[0].children[0].children[0].text == 'd'
+    assert line3.children[0].children[0].children[0].text == 'e'
+    assert line3.children[1].children[0].text == 'f'
+
+
 @assert_no_logs
 def test_linebox_text():
     page, = parse('''