Match descendant rules naming a table or cell against the content of the cell (#223)

[jakejackson1]

What was wrong

A descendant rule that names a table, row or cell never matched anything inside a cell. td img { max-width: 20mm }, table tr td img { … }, td span { color: red }, td p { … } and .cell .thumb { … } were all ignored, while img { … } and div img { … } (with the div outside the table) applied. This is the second half of #223 and mpdf#420. It is broader than images: every element in a cell was affected.

Cause

Cell content pushes no block level, so when CssMerger::mergeDescendantSelectors() merged an inline element, or a block element in a cell (which is merged with an empty $inherit), it looked in blk[blklvl]['cascadeCSS']. That is the block outside the table, and it only holds the rules left to match from there. The rules left to match inside the cell are in CssManager::$tablecascadeCSS[$tbCSSlvl], which <table>, <tr> and <td> build and which nothing inside the cell read.

Fix

Inside a table, non-table elements take their descendant rules from the current cell's cascade. That cascade is seeded from the enclosing block when the top-level table opens, so rules like div img, with the div outside the table, still apply.

Chains inside the cell itself, such as td p span, still don't match, because a block in a cell pushes no cascade level of its own. That limit applies to inline chains everywhere in mPDF, not just in cells, and is out of scope here.

Snapshot

image-border-radius changes. Its stylesheet has table.grid td p { margin: 3mm 0 0 0; font-size: 8pt; color: #606060; } for the captions under each picture, and that rule now applies: the captions are 8pt grey with a 3mm top margin, as the fixture intended. The fixture was regenerated with composer snapshot:update image-border-radius. No other snapshot changed.

Tests

tests/Mpdf/TableCellDescendantSelectorTest.php checks that tag chains, class chains, a cell class and an outer block all reach an image in a cell, and that a rule for a cell of another class does not. It also covers a table nested in the cell, and td span and td p colouring text. Seven of the nine cases fail on gravitypdf.

Split from #505, which fixes what a percentage on such an image is resolved against. Together they let td img { max-width: 100% } keep pictures inside their cells.

Closes #223 (together with #505).

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