When building a complex layout using Gutenberg, the most frustrating issue is to select a block or its parent. It is rare occurrence (at least in my experience) that you can select a block on first click without the help of the breadcrumbs at the bottom of the editor.

To solve this problem we can add an outline to each block to clearly show nested blocks and click on the appropriate one.

First we need to enqueue the style file.
Add this code to functions.php in your theme or main file in your plugin:

add_action('enqueue_block_editor_assets', function () {
    wp_enqueue_style('outline-style', plugin_dir_url(__FILE__) . "/outline-style.css", false, '1.0', 'all');
});

Then add this style inside oytline.style.css (code is presented in SCSS, make sure to convert to CSS before using it):

.wp-block {
    outline: 1px solid transparent;
    outline-offset: 7px;
    transition: all 0.2s;
    &:hover {
        outline-color: lightgray;
    }
    .wp-block {
        outline-offset: 2px;
    }
}

The code will add an outline around each block but there is still an issue of identifying the name of the block especially when using custom blocks.

We can modify the style a little bit to add the title of the block on the right top corner (code in SCSS):

.wp-block {
    outline: 1px solid transparent;
    outline-offset: 7px;
    transition: all 0.2s;
    &:hover {
        outline-color: lightgray;

        &::before {
            content: attr(data-type);
            padding: 0.3em 0.6em;
            font-size: 0.8rem;
            position: absolute;
            color: black;
            font-family: 'Trebuchet MS', 'Lucida Grande', 'Lucida Sans Unicode', 'Lucida Sans', Tahoma, sans-serif;
            top: 0;
            right: 0;
            left: initial !important;
            background-color: lightgray;
            transition: all 0.2s;
            pointer-events: none;
            z-index: 5;
            font-weight: normal;
        }
    }

    .wp-block {
        outline-offset: 2px;
    }
}

While this works fine there might be further issues that uses face. For example, some users want to always display outline (not just on hover), change the color, opacity or not display it at all.

To fix these issues I developed a WordPress plugin that enables users to change outline settings (like color, opacity and style) as well as show/hide the outline.

You can find the plugin here:
https://wordpress.org/plugins/editor-block-outline/