How to use the cmd+w shortcut without closing a Fluid app?
Yup.

Quit your Fluid.app, and then follow these steps:


Go to System Preferences » Keyboard » Shortcuts » App Shortcuts.
Click the "+" 
In the pop-up window, change "Application" from "All Applications" to "Your Fluid App.app"
Set the Menu Title to "Close Tab"
Set the Keyboard Shortcut to something else like ⌘⌥W
Click "Add"


Then repeat steps 2-6 for "New Tab", like so:


Click the "+"
In the pop-up window, change "Application" from "All Applications" to "Your Fluid App.app"
Set the Menu Title to "New Tab"
Set the Keyboard Shortcut to something else, like ⌘⌥T
Click "Add"


Go to Coder.com and use their keyboard settings to set Close Tab and New Tab to ⌘W and ⌘T