Re-add hack to avoid floating points errors

Fix #325 and shouldn't reopen #288. Now that fac5ee9 fixes line-cutting
bug when drawing, we can use a much lower relative tolerance inspired
from PEP 485 (1e-9 instead of 1e-3).

Tests have been added with random values, as results highly depend on
the version of Pango used and on hinting properties depending on the
system used to launch the tests. They are probably longer than required,
but they try hard to prevent #288 and #325 from coming back.

weasyprint/tests/test_layout.py

@@ -5713,3 +5713,50 @@ def layout(gradient_css, type_='radial', init=(),
            init=(450, 100, 0, 450 * sqrt2), scale_y=200/450)
     layout('radial-gradient(farthest-corner at 40px 210px, blue, lime)',
            init=(40, 210, 0, 360 * sqrt2), scale_y=210/360)
+
+
+@assert_no_logs
+def test_shrink_to_fit_floating_point_error():
+    """Test that no floating point error occurs during shrink to fit.
+
+    See bugs #325 and #288, see commit fac5ee9.
+
+    """
+    for margin_left in range(1, 10):
+        for font_size in range(1, 10):
+            page, = parse('''
+                <style>
+                    @page { size: 100000px 100px }
+                    p { float: left; margin-left: 0.%iin; font-size: 0.%iem;
+                        font-family: "ahem" }
+                </style>
+                <p>this parrot is dead</p>
+            ''' % (margin_left, font_size))
+            html, = page.children
+            body, = html.children
+            p, = body.children
+            assert len(p.children) == 1
+
+    letters = 1
+    for font_size in (1, 5, 10, 50, 100, 1000, 10000):
+        while True:
+            page, = parse('''
+                <style>
+                    @page { size: %i0pt %i0px }
+                    p { font-size: %ipt; font-family: "ahem" }
+                </style>
+                <p>mmm <b>%s a</b></p>
+            ''' % (font_size, font_size, font_size, 'i' * letters))
+            html, = page.children
+            body, = html.children
+            p, = body.children
+            assert len(p.children) in (1, 2)
+            assert len(p.children[0].children) == 2
+            text = p.children[0].children[1].children[0].text
+            print(font_size, text, letters)
+            assert text
+            if text.endswith('i'):
+                letters = 1
+                break
+            else:
+                letters += 1

weasyprint/text.py

@@ -426,6 +426,14 @@ def split_first_line(text, style, hinting, max_width, line_width):
 
     if text_wrap:
         max_width = None
+    elif max_width is not None:
+        # In some cases (shrink-to-fit result being the preferred width)
+        # this value is coming from Pango itself,
+        # but floating point errors have accumulated:
+        #   width2 = (width + X) - X   # in some cases, width2 < width
+        # Increase the value a bit to compensate and not introduce
+        # an unexpected line break. The 1e-9 value comes from PEP 485.
+        max_width *= 1 + 1e-9
 
     # Step #1: Get a draft layout with the first line
     layout = None