Don't set QT_HARFBUZZ with Qt >= 5.4

Fixes #1948

qutebrowser/misc/earlyinit.py

@@ -36,6 +36,7 @@ import traceback
 import signal
 import operator
 import importlib
+import pkg_resources
 try:
     import tkinter
 except ImportError:
@@ -141,6 +142,16 @@ def init_faulthandler(fileobj=sys.__stderr__):
         faulthandler.register(signal.SIGUSR1)
 
 
+def _qt_version():
+    """Get the running Qt version.
+
+    Needs to be in a function so we can do a local import easily (to not import
+    from QtCore too early) but can patch this out easily for tests.
+    """
+    from PyQt5.QtCore import qVersion
+    return pkg_resources.parse_version(qVersion())
+
+
 def fix_harfbuzz(args):
     """Fix harfbuzz issues.
 
@@ -161,6 +172,8 @@ def fix_harfbuzz(args):
     - On Qt 5.3.1 this bug is fixed and the old engine will be the more stable
       one again.
 
+    - On Qt 5.4 the new engine is the default and most bugs are taken care of.
+
     IMPORTANT: This needs to be done before QWidgets is imported in any way!
 
     WORKAROUND (remove this when we bump the requirements to 5.3.1)
@@ -169,7 +182,6 @@ def fix_harfbuzz(args):
         args: The argparse namespace.
     """
     from qutebrowser.utils import log
-    from PyQt5.QtCore import qVersion
     if 'PyQt5.QtWidgets' in sys.modules:
         msg = "Harfbuzz fix attempted but QtWidgets is already imported!"
         if getattr(sys, 'frozen', False):
@@ -177,12 +189,14 @@ def fix_harfbuzz(args):
         else:
             log.init.warning(msg)
     if sys.platform.startswith('linux') and args.harfbuzz == 'auto':
-        if qVersion() == '5.3.0':
+        if _qt_version() == pkg_resources.parse_version('5.3.0'):
             log.init.debug("Using new harfbuzz engine (auto)")
             os.environ['QT_HARFBUZZ'] = 'new'
-        else:
+        elif _qt_version() < pkg_resources.parse_version('5.4.0'):
             log.init.debug("Using old harfbuzz engine (auto)")
             os.environ['QT_HARFBUZZ'] = 'old'
+        else:
+            log.init.debug("Using system harfbuzz engine (auto)")
     elif args.harfbuzz in ['old', 'new']:
         # forced harfbuzz variant
         # FIXME looking at the Qt code, 'new' isn't a valid value, but leaving