Fix computed values on table element overwritten after first page in running tables

[kygoh]

This is a naive fix using "ghost" style properties until a proper fix can be implemented.

The comment before the computed values are overwritten is probably a hint for a proper fix but unable to grasp the logic of table wrapper box and table box yet:

WeasyPrint/weasyprint/formatting_structure/build.py

Lines 987 to 989 in 3f6775a

	# Non-inherited properties of the table element apply to one
	# of the wrapper and the table. The other get the initial value.
	# TODO: put this in a method of the table object

Note: This will fix #2013

[liZe]

Thanks for this pull request!

The comment before the computed values are overwritten is probably a hint for a proper fix but unable to grasp the logic of table wrapper box and table box yet

For now, just as for #1278, I’d prefer to keep the bug than to add a fake value in the style dictionary. We can keep this PR open as a draft until we find the source of the problem and fix it in a clean and solid way.

[kygoh]

the source of the problem

The following computed values on table element are overwritten by initial values in the first page:

WeasyPrint/weasyprint/css/properties.py

Lines 281 to 308 in 6e835e2

	# https://www.w3.org/TR/CSS21/tables.html#model
	# See also https://lists.w3.org/Archives/Public/www-style/2012Jun/0066.html
	# Only non-inherited properties need to be included here.
	TABLE_WRAPPER_BOX_PROPERTIES = {
	'bottom',
	'break_after',
	'break_before',
	'break_inside',
	'clear',
	'counter_increment',
	'counter_reset',
	'counter_set',
	'float',
	'left',
	'margin_top',
	'margin_bottom',
	'margin_left',
	'margin_right',
	'opacity',
	'overflow',
	'position',
	'right',
	'top',
	'transform',
	'transform_origin',
	'vertical_align',
	'z_index',
	}

From https://lists.w3.org/Archives/Public/www-style/2012Jun/0066.html:

Elements with 'display: table' generate two boxes: a table wrapper box and a table box.

and in https://www.w3.org/TR/CSS21/tables.html#model, these computed values

on the table element are used on the table wrapper box and not the table box; all other values of non-inheritable properties are used on the table box and not the table wrapper box. (Where the table element's values are not used on the table and table wrapper boxes, the initial values are used instead.)

In running tables, the table wrapper box and table box will be generated for every page. However for subsequent pages, the above computed values for the table element have been overwritten by initial values - it appears that table box computed values are copied in subsequent pages.

[kygoh]

Previously box style was copied as well:

WeasyPrint/weasyprint/formatting_structure/build.py

Line 629 in 45c8158

table = box.copy_with_children(row_groups, copy_style=True)

[kygoh]

The box.copy_with_children method in the following code appears to be the source of the problem:

WeasyPrint/weasyprint/formatting_structure/build.py

Line 972 in 6e835e2

table = box.copy_with_children(row_groups)

It does not make a copy of box.style for table.style but appears to be referencing box.style. Thus, changing table.style[name] is actually changing box.style[name].

[liZe]

It does not make a copy of box.style for table.style but appears to be referencing box.style. Thus, changing table.style[name] is actually changing box.style[name].

That’s a recurring problem in WeasyPrint, see for example #950 and #497.

Copying styles was an easy solution, but it was really slow and required a lot of memory (and caused different bugs). That’s why the current solution is to avoid style edition when possible, and to copy it only when it’s necessary. It’s a bit dangerous, but the performance improvement is worth the risk.

The goal is to freeze style dictionaries to forbid all editions. But unfortunately, the few cases left are not easy to fix.

Where is the style modified? We could try to take care of this special case instead of always copying it.

[kygoh]

Where is the style modified?

WeasyPrint/weasyprint/formatting_structure/build.py

Lines 990 to 992 in 6e835e2

	for name in properties.TABLE_WRAPPER_BOX_PROPERTIES:
	wrapper.style[name] = table.style[name]
	table.style[name] = properties.INITIAL_VALUES[name]

[liZe]

Where is the style modified?

Oh, of course! 😁

If it’s OK for you, I’ll keep 3926136 and merge this pull request.

[liZe]

Thanks a lot for your work!