Filter pages using tags (when some pages have multiple tags)

The CMS Questions
1

I have some case studies. Each case study is tagged with a particular category. Some case studies are tagged with more than one category. Here’s my blueprint:

title: Case study pages

columns:
	main:
		width: 2/3
		
		fields: 
			category:
				label: Services
				type: tags
				accept: options
				options:
					- Branding
					- Digital design
					- Graphic design

I’m trying to follow this video, https://www.youtube.com/watch?v=ue15rQVQCqg&list=PLTep5U-3mg9EfgnQ08XDRs4vSOmhw2JWz&index=13

Is it even possible to filter pages, when some have more than one tag?

Here’s my snippet:

<?php

	$filterBy = get('filter');

	$projects = page('portfolio')
		->children()
		->listed()
		->when($filterBy, function($filterBy) {
			return $this->filterBy('category', $filterBy);
		})
?>

<ul class="portfolio">

	<?php foreach ( $projects as $project): ?>
	<li>
		<a href="<?= $project->url() ?>">
			<?php if ($image = $project->portfolio_thumbnail()->toFile()): ?>

				<picture>
					<source
						srcset="<?= $image->srcset('folio-thumbnails-webp') ?>"
						sizes="(max-width: 1023px) 100vw, (max-width: 2240px) 25vw, 480px"
						type="image/webp"
					>
					
					<img
						loading="lazy"
						alt="<?= $image->alt() ?>"
						src="<?= $image->resize(1120)->url() ?>"
						srcset="<?= $image->srcset('folio-thumbnails') ?>"
						sizes="(max-width: 1023px) 100vw, (max-width: 2240px) 25vw, 480px"
						width="<?= $image->width() ?>"
						height="<?= $image->height() ?>"
					>
				</picture>

			<?php endif ?>
			<p><?= $project->heading() ?><br><span class="project-title"><?= $project->subheading() ?></span></p>
			<p><?= $project->category() ?></p>
		</a>
	</li>
	<?php endforeach ?>
</ul>

But when I try the filter in the browser URL:
http://localhost:8888/current-site3/about?filter=branding

The list of pages disappear.

I probably need the ‘,’ comma separation to separate the tags, but I don’t know where to put it.

2

Exactly, here

return $this->filterBy('category', $filterBy, ',');

see $pages->filterBy() | Kirby CMS

3

Thank you.