Detect softTabs/hardTabs when an EditSession is created

src/app/buffer.coffee

@@ -352,6 +352,12 @@ class Buffer
         return row unless @isRowBlank(row)
     null
 
+  usesSoftTabs: ->
+    for line in @getLines()
+      if match = line.match(/^\s/)
+        return match[0][0] != '\t'
+    undefined
+
   logLines: (start=0, end=@getLastRow())->
     for row in [start..end]
       line = @lineForRow(row)

src/app/edit-session.coffee

@@ -39,9 +39,9 @@ class EditSession
   softTabs: true
   softWrap: false
 
-  constructor: ({@project, @buffer, @tabLength, @autoIndent, @softTabs, @softWrap }) ->
+  constructor: ({@project, @buffer, @tabLength, @autoIndent, softTabs, @softWrap }) ->
     @id = @constructor.idCounter++
-    @softTabs ?= true
+    @softTabs = @buffer.usesSoftTabs() ? softTabs ? true
     @languageMode = new LanguageMode(this, @buffer.getExtension())
     @displayBuffer = new DisplayBuffer(@buffer, { @languageMode, @tabLength })
     @tokenizedBuffer = @displayBuffer.tokenizedBuffer