Meld: How to open /usr [Mac OS 10.13.2, Meld 3.19.0]
After choosing either File or Folder comparison, when I click the button to choose a file/folder and select the root folder (label of the drive mounted as /), the selection of folders is limited to:  

Applications
Library
mounts (I've changed permissions for this)
System
User Information
Users


However, because none of the other folders are listed, I cannot select /usr.  I suspect this has something to do with GTK+, and an earlier Meld version worked fine.

Do I need to hold down special keys (like option) or do something else to allow this?

As a workaround, I created a symlink from my home folder to the folder in /usr/local/ that I needed to analyse.  However I don't want to change file/folder permissions just for this.
You just have to hit cmdshift. once to get all invisible files and folders (either hidden by a prepended dot or the hidden attribute) in the "open" dialog. This persists even after navigating to other folders in the same window.

To see and select another invisible file or folder to complete the comparison you have to hit the shortcut again.

Before the shortcut:



After the shortcut: