Warn user about buggy libraries

Some bugs are out of our reach to fix, but can still impact the user
considerably. Because losing progress always hurts, we want to make
the user aware of the risks before any tears are shed. (PR #612)

manuskript/main.py

@@ -3,17 +3,49 @@
 import faulthandler
 import os
 import sys
+import re
 
 import manuskript.ui.views.webView
+from PyQt5.Qt import qVersion
 from PyQt5.QtCore import QLocale, QTranslator, QSettings
 from PyQt5.QtGui import QIcon
-from PyQt5.QtWidgets import QApplication, qApp
+from PyQt5.QtWidgets import QApplication, qApp, QMessageBox
 
 from manuskript.functions import appPath, writablePath
 from manuskript.version import getVersion
 
 faulthandler.enable()
 
+def warnAboutBuggyLibraries(app):
+    """Some bugs are out of our reach to fix. The user needs to be warned and perhaps take action themselves."""
+
+    # (Py)Qt 5.11 and 5.12 have a bug that can cause crashes when simply setting up
+    # various UI elements. This has been reported and verified to happen in the
+    # Export (Compile) screen, but due to the nature of the bug, we cannot be sure
+    # it won't cause random crashes in other parts of the application. (PR-612)
+
+    if re.match("^5\\.1[12](\\.?|$)", qVersion()):
+        warning1 = "The version of PyQt you are using ({}) is known to have a bug that can cause Manuskript to crash."
+        warning2 = "It is recommended that you upgrade to the latest version of PyQt."
+
+        # Don't translate for debug log.
+        print("WARNING:", warning1.format(qVersion()), warning2)
+
+        msg = QMessageBox(QMessageBox.Warning,
+            app.tr("You may experience crashes."),
+            "<p><b>" +
+                app.tr(warning1).format(qVersion()) +
+            "</b></p>" +
+            "<p>" +
+                app.tr(warning2) +
+            "</p>",
+            QMessageBox.Ignore | QMessageBox.Abort)
+
+        # Dialogs without a choice on them are just asking to be ignored...
+        # But with the option to 'Abort'...? Maybe someone will actually read it.
+        if msg.exec() == QMessageBox.Abort:
+            sys.exit(1)
+
 def prepare(tests=False):
     app = QApplication(sys.argv)
     app.setOrganizationName("manuskript"+("_tests" if tests else ""))
@@ -92,6 +124,8 @@ def prepare(tests=False):
     return app, MW
 
 def launch(app, MW = None):
+    warnAboutBuggyLibraries(app)
+
     if MW is None:
         from manuskript.functions import mainWindow
         MW = mainWindow()