Revert "always disable xvfb" - it is still needed for Electron

This reverts commit 058679f.

cli/lib/exec/xvfb.js

@@ -1,8 +1,11 @@
+const os = require('os')
 const Promise = require('bluebird')
 const Xvfb = require('@cypress/xvfb')
+const { stripIndent } = require('common-tags')
 const debug = require('debug')('cypress:cli')
 const debugXvfb = require('debug')('cypress:xvfb')
 const { throwFormErrorText, errors } = require('../errors')
+const util = require('../util')
 
 const xvfb = Promise.promisifyAll(new Xvfb({
   timeout: 30000, // milliseconds
@@ -44,9 +47,33 @@ module.exports = {
   },
 
   isNeeded () {
-    debug('stubbing isNeeded false %o', new Error().stack)
+    if (os.platform() !== 'linux') {
+      return false
+    }
+
+    if (process.env.DISPLAY) {
+      const issueUrl = util.getGitHubIssueUrl(4034)
+
+      const message = stripIndent`
+        DISPLAY environment variable is set to ${process.env.DISPLAY} on Linux
+        Assuming this DISPLAY points at working X11 server,
+        Cypress will not spawn own Xvfb
+
+        NOTE: if the X11 server is NOT working, Cypress will exit without explanation,
+          see ${issueUrl}
+        Solution: Unset the DISPLAY variable and try again:
+          DISPLAY= npx cypress run ...
+      `
+
+      debug(message)
+
+      return false
+    }
+
+    debug('undefined DISPLAY environment variable')
+    debug('Cypress will spawn its own Xvfb')
 
-    return false
+    return true
   },
 
   // async method, resolved with Boolean