Help with max-height of slideshow

I made the suggested change to make Slideshow Image Adjustment display “Show whitespace to the sides for narrower images” but that just hides the bottom half of the portrait image.

The problem is that your theme has got conflicting styles that are messing with Photonic’s. Your theme is doing this:

img, video, object {
    max-width: 100%;
    height: auto!important;
}

That setting of height: auto!important is overriding the calculation by Photonic. You will have to work with your theme’s author to figure out the impact of removing the !important directive. This will mess with pretty much every type of layout that Photonic has.

If possible, it would be good to show an image full size in a lightbox, but I’ve not worked out where the correct option is.

There isn’t an option for this, because a lightbox cannot work with a slideshow. Lightboxes are meant for non-slideshow layouts such as square thumbnails, or justified grids. A slideshow shows one image at a time as does a lightbox, hence coupling the two introduces redundant functionality.

It should be possible to use the max-height CSS property somewhere, but I’m unclear where to enter it.

To be compatible with all of WP, Photonic doesn’t define its own location to enter custom CSS. Instead you can use the standard location under Appearance → Customize → Additional CSS. That being said, as written above, your problem is with your theme here, not with Photonic or any custom CSS.

On a side note, is there a way to make next/prev button more visible on landscape black and white photos?

You can add this custom CSS. Modify the colours to suit your requirement:

.splide__arrow { background: #a00; }
.splide__arrow path { fill: #fff; }

I know exactly which I setting it is, I change it and save, reload the page… no change. Back to the relevant settings page, reload it… it still hasn’t changed. It drives me crazy! The setting in question is how to display the image title, I’ve reviewed the option in the gallery itself and in the default settings.

While I cannot help troubleshoot the behaviour of the back-end, note that all options don’t apply to the slideshow. It is not possible to show the title below the photo – this would mess with the slideshow sizing. The closest you can get is to cover the lower portion of the photo. If you pick a setting to display the title below the photos, Photonic will use the global default of “Tooltip”.

In Google Photos settings, I select “Always use the photo title, even if blank”.

This just changes the content of the title, not its appearance.

On the Galleries themselves, I make sure it’s all default setting everywhere. The title should be “Cover the lower portion always” but no matter how often I save it, it just remains stuck on “Using a Javascript tooltip”.

Can you try setting this explicitly on the slideshow instead of using the default? So, in the last step (where you are building the gallery), there is an option for “How do you want the title?” – you can set it to “Cover the lower portion always”. Basically, if you are able to see the shortcode, you would be adding the option, title_position="slideup-stick" in the shortcode. I have verified that this works.

I’ve tried that, this is where it doesn’t seem to save, so incorrect shortcode continues to be shown.

Are you using the Classic Editor or Gutenberg? If you are using the Classic Editor, you can actually go to the “Text” mode instead of the “Visual” mode. That will let you directly edit the shortcode without launching the wizard.

if this conflicting code can’t be resolved

Add this to your Custom Styles and see if it makes a difference:
.photonic-stream img { height: auto; }

If it does, then you don’t have to immediately fix your theme, but you will continue to face a similar problem any time you work with a plugin that deals with custom image sizes.