Never modify files in the folder "concrete"!

Go to the "blocks" folder
Create a folder "autonav"
Another one "templates"
Copy the "bad" view.php to the templates folder you've just created
Rename it to whatever you want

Edit the page (or page defaults) where the problems occurs, click the autonav block, select the custom template.

Now you can modify your autonav template without getting problems in the future

But doesn't that possibly cause problems as well? What happens when a new version (with additional features) of the autonav block is released? Don't I lose out on the new features when I upgrade?

I'm curious what that line is for anyway. Shouldn't it only be present when the page is in edit mode?

This is why I wrote that you should never modify files within "concrete". No problems with updates as long as they don't change their APIs and this usually doesn't happen every day

However to answer your other question, if you copy

concrete/blocks/autonav/templates/header_menu.php

to

blocks/autonav/templates/header_menu.php

and make your changes to the second path (the one in the local blocks directory) it should override the one in the concrete/ directory...but whenever c5 is upgraded you should still get all the nice new functionality of the autonav block, because you're only overriding one specific template.

True, you're overriding all cases of that template, but since it sounds like you're using a custom theme that shouldn't be a problem.

My hack was just to set:

.ccm-spacer {
height: 0;
}

in my theme's main.css

I think the height is already set to 0 in a concrete/css file

ccm.forms.css -- line 45
div.ccm-spacer {font-size: 0px; line-height: 0px; clear: both; height: 0px}

I am new to concrete5 and using version 5.4.1.1 so maybe this has changed since this post was made

Or add this to your theme's CSS file:

.ccm-spacer {
display:none!important;
}

EDIT: Make sure you define an area that the class will be changed within or you'll mess up the layout of the CMS.

eg:

#header .ccm-spacer {
display:none!important;
}

Now only ccm-spacer in the div with id="heading" will be hidden and all others will display. "header" could be anything, it could also be a class and not an id.