Hey there -
I have the same exact issue. I’ve been reading all these great blog post that rave about this feature and how multiple selections is a huge timesaver. They all state that Command-D initiates multiple selections. However when I try to use it, nothing happens. When I use my mouse to go up to the Find menu and select Quick Add Next, it works. But the keyboard shortcut does nothing. Like you I tried to manually add a keyboard shortcut from the Mac’s Keyboard System Preferences. I now see a Command-D (⌘D) next to the Quick Add Next menu item, but the keyboard shortcut still does nothing.
In my Default (OSX).sublime-keymap I see the following line:
{ "keys": "super+d"], "command": "find_under_expand" },
I have nothing in my Key Bindings - User file that overrides this keyboard shortcut.
How is it that Command-D works for so many people?
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