Always suggest non-negative indent levels

Close atom#668

CHANGELOG.md

@@ -1,3 +1,5 @@
+* Fixed: Error when inserting newlines in CSS
+
 * Improved: cmd-n now opens a new tab and cmd-shift-n now opens a new window.
 * Added: Inspect Element context menu
 * Fixed: Save As dialog now defaults to directory path of current editor

spec/app/language-mode-spec.coffee

@@ -341,3 +341,14 @@ describe "LanguageMode", ->
 
         fold2 = editSession.lineForScreenRow(5).fold
         expect(fold2).toBeFalsy()
+
+  describe "css", ->
+    beforeEach ->
+      atom.activatePackage('source-tmbundle', sync: true)
+      atom.activatePackage('css-tmbundle', sync: true)
+      editSession = project.open('css.css', autoIndent: true)
+
+    describe "suggestedIndentForBufferRow", ->
+      it "does not return negative values (regression)", ->
+        editSession.setText('.test {\npadding: 0;\n}')
+        expect(editSession.suggestedIndentForBufferRow(2)).toBe 0

src/app/language-mode.coffee

@@ -232,7 +232,7 @@ class LanguageMode
     return desiredIndentLevel unless decreaseIndentRegex = @decreaseIndentRegexForScopes(scopes)
     desiredIndentLevel -= 1 if decreaseIndentRegex.test(currentLine)
 
-    desiredIndentLevel
+    Math.max(desiredIndentLevel, 0)
 
   # Calculate a minimum indent level for a range of lines excluding empty lines.
   #