[gravitypdf-bot]

Fixes #742

Found while working on #552

Stacked on #723

Problem

A table header or footer that is drawn again on later pages raises Undefined array key "gradient" warnings on each of those pages when one of its cells or rows has a background-image loaded from a URL. The image itself is still drawn. The warnings are the only fault, but a strict error handler turns them into a failed document.

$mpdf = new \Mpdf\Mpdf();
$rows = str_repeat('<tr><td>Row</td><td>Row</td></tr>', 80);
$mpdf->WriteHTML('<table repeat_header="1">
<thead>
	<tr><th style="background-image: url(bg.jpg)">Cell image</th><th>Plain</th></tr>
	<tr style="background-image: url(bg.jpg)"><th>Row image</th><th>Row image</th></tr>
</thead>
<tbody>' . $rows . '</tbody></table>');

	First page	Repeated header or footer
Before	image, no warning	image, Undefined array key "gradient" per cell or row
After	image, no warning	image, no warning

Cause

TableHeaderFooter() reads $bg['gradient'] for every background layer of a header or footer cell and row without checking that the key exists. A layer from a URL has image_id and no gradient key. _tableWrite() draws the same cell on the table's first page, and it checks the key with isset() first.

This branch is stacked on #723 because #723 rewrites both of these lines into a loop over background layers. The unguarded read is still there after that rewrite, now as $bg['gradient'].

Fix

Both reads are now !empty($bg['gradient']), the same check that isset() && truthy makes elsewhere in the file.

Legacy mode

This is a fix for a warning, and it applies in both cssModes. Nothing on the page changes in either mode.

Tests

• tests/Mpdf/RepeatedTableHeaderBackgroundTest.php renders a three-page table whose header cell, header row, footer cell or footer row has a background image, in both CSS modes (8 cases). It uses the PageStreams trait's assertDrawsSilently() to assert that nothing is raised, and it asserts that a pattern is painted on every page. All 8 cases fail against the src/ on Draw every layer of a multiple background (#609) #723 with the warnings from lines 13361 and 13546, and they pass with the fix.
• No snapshot test. The image was drawn before and is drawn the same way after, so no page changes and a snapshot could not tell the two apart.

Checks

• composer test: OK (12770 tests, 31 skipped, none of them snapshots; the snapshot group alone passes 219/219, none skipped)
• composer cs: clean
• PHPStan: same 23 errors as on gravitypdf, none new

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