Change multi-cursor editing shortcut - sublimetext3

Question

Right now in order to perform a multi-cursor edit in SublimeText3 I press Ctrl and click the places I want to edit.
I want to change this so I press Alt (instead of 'Ctrl') and then click the places I want to edit.
I feeling like it should be somewhere in Preferences -> Key Bindings, but I was not able to find the option.
How can I change this key binding?

Answer 1

As the bindings here are for mouse buttons and not for keyboard keys, the actual bindings are not stored in sublime-keymap files and are instead stored in sublime-mousemap files. This is a format similar to a sublime-keymap file, but it includes the ability to map a command to mouse buttons, click counts and modifier keys.
Note: unlike a key binding, it's not possible to include a context key, so mouse bindings are an all-or-nothing affair; you can't set some to only apply in some circumstances and not others, for example.
In order to modify the mouse bindings you need to follow the same general procedure as you would for a key binding, which is to create a file with the appropriate name in your User package that includes your modified bindings. There aren't any default menu items or command palette entries that allow you to edit the mouse mappings, so you need to do this manually.
Note: In the following file names, $PLATFORM is one of Windows, OSX or Linux depending on what platform you're on (case is important). Your question doesn't mention which platform you're on, so the instructions below assume Windows; change the platform as needed for your own uses. On OSX your bindings can include super to represent the command key, as they can in key bindings.
Where the name of a key binding file is Default ($PLATFORM).sublime-keymap,the appropriate file for mouse bindings is Default ($PLATFORM).sublime-mousemap. Additionally, like most resource files that set the default behaviour, the base files are stored in the Default package.
If you use the View Package File command palette entry to open Default/Default (Windows).sublime-mousemap, you can see all of the default mouse bindings. There are two bindings in particular that you probably want to work with here, and they're near the top of the file in the "Basic drag select" section:
{
"button": "button1", "count": 1, "modifiers": ["ctrl"],
"press_command": "drag_select",
"press_args": {"additive": true}
},
{
"button": "button1", "count": 1, "modifiers": ["alt"],
"press_command": "drag_select",
"press_args": {"subtractive": true}
},
The first binding says that when the Left mouse button (button1) is clicked a single time while the ctrl key is pressed, the drag_select command should be executed with an argument that indicates the selection should be added to.
Similarly, the second binding uses alt instead of ctrl and executes the same command, telling it to subtract from the selection instead of adding to it.
Thus, Ctrl+Left Click adds a caret (and any text you drag over) to the selection while Alt+Left Click does the reverse and removes carets and text you drag over.
To effect the change you want, create a file with the following contents and save it as Default (Windows).sublime-mousemap in your User package (you can use Preferences > Browse Packages to find where that is).
Note: If such a file already exists, add the two bindings here to the existing file instead of saving over it; in that case don't include the [ and ] characters, but make sure you paste the bindings in between the ones in the existing file along with the other bindings.
[
{
"button": "button1", "count": 1, "modifiers": ["alt"],
"press_command": "drag_select",
"press_args": {"additive": true}
},
{
"button": "button1", "count": 1, "modifiers": ["ctrl"],
"press_command": "drag_select",
"press_args": {"subtractive": true}
},
]
This is the two bindings from above, but with the modifiers swapped. That makes the Alt key add selections and Ctrl remove selection, effectively reversing their actions.
If you like you only need to add the binding for alt. However if you do that, the binding for ctrl will still exist, meaning you will have two bindings for adding a selection and no bindings for removing a selection. That may or may not be of concern depending on how much you rely on that particular binding.