Don't use stylesheets anymore for QTabWidget.

When using stylesheets there, all children (i.e. the QWebView) use
QStyleSheetStyle as well, which means the bug at [1] will annoy us.

We had to write our own font parsing as we can't use stylesheets, but at least
the bug is gone...

https://bugreports.qt-project.org/browse/QTBUG-40263

qutebrowser/config/configdata.py

@@ -940,7 +940,7 @@ DATA = OrderedDict([
          "Font used in the completion widget."),
 
         ('tabbar',
-         SettingValue(types.Font(), '8pt ${_monospace}'),
+         SettingValue(types.QtFont(), '8pt ${_monospace}'),
          "Font used in the tabbar."),
 
         ('statusbar',

qutebrowser/config/conftypes.py

@@ -25,13 +25,30 @@ import os.path
 from sre_constants import error as RegexError
 
 from PyQt5.QtCore import QUrl, QStandardPaths
-from PyQt5.QtGui import QColor
+from PyQt5.QtGui import QColor, QFont
 from PyQt5.QtNetwork import QNetworkProxy
 
 import qutebrowser.commands.utils as cmdutils
 from qutebrowser.utils.misc import get_standard_dir
 
 
+QSS_FONT_REGEX = re.compile(r"""
+    ^(
+        (
+            # style
+            (?P<style>normal|italic|oblique) |
+            # weight (named | 100..900)
+            (
+                (?P<weight>[123456789]00) |
+                (?P<namedweight>normal|bold)
+            ) |
+            # size (<float>pt | <int>px)
+            (?P<size>[0-9]+((\.[0-9]+)?[pP][tT]|[pP][xX]))
+        )\                     # size/weight/style are space-separated
+    )*                         # 0-inf size/weight/style tags
+    (?P<family>[A-Za-z, "]*)$  # mandatory font family""", re.VERBOSE)
+
+
 class ValidationError(ValueError):
 
     """Exception raised when a value for a config type was invalid.
@@ -530,8 +547,45 @@ class Font(BaseType):
     typestr = 'font'
 
     def validate(self, value):
-        # We can't really validate anything here
+        if not QSS_FONT_REGEX.match(value):
-        pass
+            raise ValidationError(value, "must be a valid font")
+
+
+class QtFont(Font):
+
+    """A Font which gets converted to q QFont."""
+
+    def transform(self, value):
+        style_map = {
+            'normal': QFont.StyleNormal,
+            'italic': QFont.StyleItalic,
+            'oblique': QFont.StyleOblique,
+        }
+        weight_map = {
+            'normal': QFont.Normal,
+            'bold': QFont.Bold,
+        }
+        font = QFont()
+        match = QSS_FONT_REGEX.match(value)
+        style = match.group('style')
+        weight = match.group('weight')
+        namedweight = match.group('weight')
+        size = match.group('size')
+        family = match.group('family')
+        if style:
+            font.setStyle(style_map[style])
+        if namedweight:
+            font.setWeight(weight_map[namedweight])
+        if weight:
+            # based on qcssparser.cpp:setFontWeightFromValue
+            font.setWeight(min(int(weight) / 8, 99))
+        if size:
+            if size.lower().endswith('pt'):
+                font.setPointSizeF(float(size[:-2]))
+            elif size.lower().endswith('px'):
+                font.setPointSizeF(int(size[:-2]))
+        font.setFamily(family)
+        return font
 
 
 class Regex(BaseType):

qutebrowser/widgets/tabwidget.py

@@ -32,7 +32,6 @@ from PyQt5.QtWidgets import (QTabWidget, QTabBar, QSizePolicy, QCommonStyle,
                              QApplication)
 from PyQt5.QtGui import QIcon, QPalette, QColor
 
-from qutebrowser.config.style import set_register_stylesheet
 from qutebrowser.utils.qt import qt_ensure_valid
 import qutebrowser.config.config as config
 
@@ -42,23 +41,7 @@ PM_TabBarPadding = QStyle.PM_CustomBase
 
 class TabWidget(QTabWidget):
 
-    """The tabwidget used for TabbedBrowser.
+    """The tabwidget used for TabbedBrowser."""
-
-    Class attributes:
-        STYLESHEET: The stylesheet template to be used.
-    """
-
-    STYLESHEET = """
-        QTabWidget::pane {{
-            position: absolute;
-            top: 0px;
-        }}
-
-        QTabBar {{
-            {font[tabbar]}
-            {color[tab.bg.bar]}
-        }}
-    """
 
     def __init__(self, parent):
         super().__init__(parent)
@@ -66,7 +49,6 @@ class TabWidget(QTabWidget):
         self.setTabBar(bar)
         bar.tabCloseRequested.connect(self.tabCloseRequested)
         self.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.Fixed)
-        set_register_stylesheet(self)
         self.setDocumentMode(True)
         self.setElideMode(Qt.ElideRight)
         self.setUsesScrollButtons(True)
@@ -98,8 +80,9 @@ class TabWidget(QTabWidget):
         tabbar.refresh()
 
     @pyqtSlot(str, str)
-    def on_config_changed(self, section, _option):
+    def on_config_changed(self, section, option):
         """Update attributes when config changed."""
+        self.tabBar().on_config_changed(section, option)
         if section == 'tabbar':
             self._init_config()
 
@@ -119,6 +102,7 @@ class TabBar(QTabBar):
     def __init__(self, parent=None):
         super().__init__(parent)
         self.setStyle(TabBarStyle(self.style()))
+        self.setFont(config.get('fonts', 'tabbar'))
         self.vertical = False
 
     def __repr__(self):
@@ -141,6 +125,12 @@ class TabBar(QTabBar):
         self.setTabData(idx, color)
         self.update(self.tabRect(idx))
 
+    @pyqtSlot(str, str)
+    def on_config_changed(self, section, option):
+        """Update attributes when config changed."""
+        if section == 'fonts' and option == 'tabbar':
+            self.setFont(config.get('fonts', 'tabbar'))
+
     def mousePressEvent(self, e):
         """Override mousePressEvent to close tabs if configured."""
         button = config.get('tabbar', 'close-mouse-button')