Stop a table with no background leaving the next one painted twice

src/Mpdf.php

@@ -23469,29 +23469,27 @@ function _tableWrite(&$table, $split = false, $startrow = 0, $startcol = 0, $spl
 			/* -- END BACKGROUNDS -- */
 		}
 
-		if ($this->tableBackgrounds && $level == 1) {
+		if ($level == 1) {
 			// Nothing else paints while a page-break-inside:avoid block is being measured - see
 			// PaintDivBB() and BaseWriter::write() - and what is drawn on the page it starts on
 			// stays there when the block moves
 			if (!$this->keep_block_together) {
-				$s = $this->PrintTableBackgrounds();
+				$s = $this->tableBackgrounds ? "\n" . $this->PrintTableBackgrounds() . "\n" : '';
+
+				// The placeholder this table wrote is spent whether or not anything was put behind
+				// it. One left in place takes a copy of the next table's backgrounds as well, so a
+				// table with no background of its own used to leave the one after it painted twice
+				$placeholder = '___TABLE___BACKGROUNDS' . $this->uniqstr;
+
 				if ($this->table_rotate && !$this->processingHeader && !$this->processingFooter) {
-					$this->tablebuffer = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', '\\1' . "\n" . $s . "\n", $this->tablebuffer);
-					if ($level == 1) {
-						$this->tablebuffer = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', " ", $this->tablebuffer);
-					}
+					$this->tablebuffer = str_replace($placeholder, ' ' . $s, $this->tablebuffer);
 				} elseif ($this->bufferoutput) {
-					$this->headerbuffer = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', '\\1' . "\n" . $s . "\n", $this->headerbuffer);
-					if ($level == 1) {
-						$this->headerbuffer = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', " ", $this->headerbuffer);
-					}
+					$this->headerbuffer = str_replace($placeholder, ' ' . $s, $this->headerbuffer);
 				} else {
-					$this->pages[$this->page] = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', '\\1' . "\n" . $s . "\n", $this->pages[$this->page]);
-					if ($level == 1) {
-						$this->pages[$this->page] = preg_replace('/(___TABLE___BACKGROUNDS' . $this->uniqstr . ')/', " ", $this->pages[$this->page]);
-					}
+					$this->pages[$this->page] = str_replace($placeholder, ' ' . $s, $this->pages[$this->page]);
 				}
 			}
+
 			$this->tableBackgrounds = [];
 		}
 

tests/Mpdf/TableBackgroundTest.php

@@ -28,15 +28,14 @@ private function pages($pdf)
 	}
 
 	/**
-	 * The rectangles filled red, without the duplicates that come of the same background being
-	 * spliced in behind more than one placeholder
+	 * Every rectangle filled red, in order, including any drawn more than once
 	 */
 	private function redRectangles($pdf)
 	{
 		$matches = [];
 		preg_match_all('/' . preg_quote(self::RED_FILL, '/') . '\n([\d.\- ]+) re f/', $pdf, $matches);
 
-		return array_values(array_unique($matches[1]));
+		return $matches[1];
 	}
 
 	private function document($before, $after)
@@ -87,4 +86,54 @@ public function testATableBackgroundOutsideAnyAvoidBlockIsUnaffected()
 		$this->assertCount(1, $this->redRectangles($this->render($html)));
 	}
 
+	/**
+	 * Each table writes a placeholder for its backgrounds to be spliced in behind. A table with no
+	 * background of its own left the placeholder in the page, and the next table's backgrounds went
+	 * behind that one as well as its own.
+	 */
+	public function testATableWithNoBackgroundDoesNotLeaveTheNextOnePaintedTwice()
+	{
+		$html = '<table><tr><td>Plain table cell</td></tr></table>'
+			. '<table><tr><td bgcolor="red">Red table cell</td></tr></table>';
+
+		$this->assertCount(1, $this->redRectangles($this->render($html)));
+	}
+
+	public function testEachTableWithNoBackgroundUsedToAddAnotherCopy()
+	{
+		$html = str_repeat('<table><tr><td>Plain table cell</td></tr></table>', 5)
+			. '<table><tr><td bgcolor="red">Red table cell</td></tr></table>';
+
+		$this->assertCount(1, $this->redRectangles($this->render($html)));
+	}
+
+	/**
+	 * Painting the same rectangle twice is invisible while it is opaque; a translucent one comes out
+	 * darker than it was asked to be
+	 */
+	public function testATranslucentBackgroundIsNotDarkenedByBeingPaintedTwice()
+	{
+		$table = '<table style="background: rgba(255, 0, 0, 0.35)"><tr><td>Translucent table cell</td></tr></table>';
+
+		$alone = $this->render($table);
+		$preceded = $this->render('<table><tr><td>Plain table cell</td></tr></table>' . $table);
+
+		$this->assertSame(
+			preg_match_all('/ re f/', $alone),
+			preg_match_all('/ re f/', $preceded)
+		);
+	}
+
+	/**
+	 * The table with the background comes first here, so its placeholder was always taken out before
+	 * the plain table wrote one - this way round has never been wrong
+	 */
+	public function testATableWithNoBackgroundAfterAColouredOneIsStillUnaffected()
+	{
+		$html = '<table><tr><td bgcolor="red">Red table cell</td></tr></table>'
+			. '<table><tr><td>Plain table cell</td></tr></table>';
+
+		$this->assertCount(1, $this->redRectangles($this->render($html)));
+	}
+
 }

tests/Snapshots/TableBackgroundSnapshotTest.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace Snapshots;
+
+/**
+ * @group snapshot
+ */
+class TableBackgroundSnapshotTest extends Snapshot
+{
+	public function getId()
+	{
+		return 'table-background';
+	}
+
+	public function generatePdf()
+	{
+		ob_start();
+		?>
+		<style>
+			p.note { margin: 0 0 3mm 0; }
+
+			table.panel { border-collapse: collapse; width: 100%; margin-bottom: 4mm; }
+			table.panel td { padding: 3mm; border: 0.2mm solid #404040; }
+
+			table.plain { background: none; }
+			table.red { background: rgba(208, 32, 32, 0.35); }
+			table.blue { background: rgba(32, 64, 192, 0.35); }
+
+			td.tint { background: rgba(32, 128, 32, 0.35); }
+		</style>
+
+		<h1>mPDF</h1>
+		<h2>Table backgrounds</h2>
+
+		<p class="note">Each pair below is the same colour written twice. The second of each pair has a
+			table with no background of its own in front of it, which used to leave the pair's second
+			half painted on top of itself and so darker than the first.</p>
+
+		<table class="panel red">
+			<tr><td>Translucent red, with nothing in front of it</td></tr>
+		</table>
+
+		<table class="panel plain">
+			<tr><td>A table with no background of its own</td></tr>
+		</table>
+
+		<table class="panel red">
+			<tr><td>Translucent red again, and the same shade as the first</td></tr>
+		</table>
+
+		<table class="panel plain">
+			<tr><td>Another table with no background of its own</td></tr>
+		</table>
+
+		<table class="panel blue">
+			<tr><td>Translucent blue, one table with no background behind it</td></tr>
+		</table>
+
+		<table class="panel plain">
+			<tr><td>A third table with no background of its own</td></tr>
+		</table>
+
+		<table class="panel">
+			<tr><td class="tint">A translucent cell rather than a translucent table</td></tr>
+		</table>
+		<?php
+		$html = ob_get_clean();
+
+		$this->mpdf = new \Mpdf\Mpdf();
+		$this->mpdf->WriteHTML($html);
+	}
+}