Have show_browser() check for headless and complain

src/calibre/gui2/__init__.py

@@ -891,6 +891,7 @@ class Application(QApplication):
             if not args:
                 args = sys.argv[:1]
             args.extend(['-platformpluginpath', sys.extensions_location, '-platform', 'headless'])
+        self.headless = headless
         qargs = [i.encode('utf-8') if isinstance(i, unicode) else i for i in args]
         self.pi = plugins['progress_indicator'][0]
         QApplication.__init__(self, qargs)
@@ -1131,9 +1132,11 @@ def ensure_app():
     with _ea_lock:
         if _store_app is None and QApplication.instance() is None:
             args = sys.argv[:1]
-            if islinux or isbsd:
+            headless = islinux or isbsd
+            if headless:
                 args += ['-platformpluginpath', sys.extensions_location, '-platform', 'headless']
             _store_app = QApplication(args)
+            _store_app.headless = headless
             import traceback
             # This is needed because as of PyQt 5.4 if sys.execpthook ==
             # sys.__excepthook__ PyQt will abort the application on an

src/calibre/web/jsbrowser/browser.py

@@ -13,7 +13,8 @@ from threading import current_thread
 
 from PyQt5.QtWebKit import QWebSettings, QWebElement
 from PyQt5.QtWebKitWidgets import QWebPage, QWebView
-from PyQt5.Qt import (QObject, QNetworkAccessManager, QNetworkDiskCache,
+from PyQt5.Qt import (
+    QObject, QNetworkAccessManager, QNetworkDiskCache, QCoreApplication,
     QNetworkProxy, QNetworkProxyFactory, QEventLoop, QUrl, pyqtSignal,
     QDialog, QVBoxLayout, QSize, QNetworkCookieJar, Qt, pyqtSlot, QPixmap)
 
@@ -681,6 +682,8 @@ class Browser(QObject, FormsMixin):
         '''
         Show the currently loaded web page in a window. Useful for debugging.
         '''
+        if getattr(QCoreApplication.instance(), 'headless', False):
+            raise RuntimeError('Cannot show browser when running in a headless Qt application')
         view = BrowserView(self.page)
         view.exec_()