Add window.print() support for Qt 5.12

See #3992

doc/changelog.asciidoc

@@ -81,6 +81,7 @@ Fixed
 - Completion highlighting now works again on Qt 5.11.3 and 5.12.1.
 - The outdated header `X-Do-Not-Track` is no longer sent.
 - A javascript error on page load when using Qt 5.12 was fixed.
+- `window.print()` works with Qt 5.12 now.
 
 v1.5.2
 ------

qutebrowser/browser/webengine/webenginetab.py

@@ -927,6 +927,10 @@ class _WebEngineScripts(QObject):
             utils.read_file('javascript/webelem.js'),
             utils.read_file('javascript/caret.js'),
         )
+        if not qtutils.version_check('5.12'):
+            # WORKAROUND for Qt versions < 5.12 not exposing window.print().
+            # Qt 5.12 has a printRequested() signal so we don't need this hack
+            # anymore.
             self._inject_early_js('js',
                                   utils.read_file('javascript/print.js'),
                                   subframes=True,
@@ -1418,15 +1422,20 @@ class WebEngineTab(browsertab.AbstractTab):
         if not qtutils.version_check('5.11.1', compiled=False):
             self.settings.update_for_url(url)
 
+    @pyqtSlot()
+    def _on_print_requested(self):
+        """Slot for window.print() in JS."""
+        try:
+            self.printing.show_dialog()
+        except browsertab.WebTabError as e:
+            message.error(str(e))
+
     @pyqtSlot(usertypes.NavigationRequest)
     def _on_navigation_request(self, navigation):
         super()._on_navigation_request(navigation)
 
         if navigation.url == QUrl('qute://print'):
-            try:
+            self._on_print_requested()
-                self.printing.show_dialog()
-            except browsertab.WebTabError as e:
-                message.error(str(e))
             navigation.accepted = False
 
         if not navigation.accepted or not navigation.is_main_frame:
@@ -1472,6 +1481,11 @@ class WebEngineTab(browsertab.AbstractTab):
             self._on_proxy_authentication_required)
         page.contentsSizeChanged.connect(self.contents_size_changed)
         page.navigation_request.connect(self._on_navigation_request)
+        try:
+            page.printRequested.connect(self._on_print_requested)
+        except AttributeError:
+            # Added in Qt 5.12
+            pass
 
         view.titleChanged.connect(self.title_changed)
         view.urlChanged.connect(self._on_url_changed)