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You may have heard the terms “fluid” or “liquid” layout. This layout is completely percentage based, so image thumbnails will always flow to completely fill the available space.
However, this is a very limited-functionality layout. It does not support the gallery's accessory features in the thumbnail view, nor styling such as borders, margins, etc. You may still use accessory features like Google Maps, hi-res image downloads, e-commerce, etc. in the large image view when using Highslide, but these features are unavailable on the grid.
Still, for basic presentation galleries that don't need the extra features, the fluid squares layout is a viable choice.