qt BalanceToolButton: better sizing on high DPI screens

specifically, on Windows, with display scaling > 100%, the pie btn was too small (not scaled)

electrum/gui/qt/balance_dialog.py

@@ -102,9 +102,8 @@ class BalanceToolButton(QToolButton, PieChartObject):
 
     def __init__(self):
         QToolButton.__init__(self)
-        self.size = max(18, font_height())
         self._list = []
-        self.R = QRect(6, 3, self.size, self.size)
+        self._update_size()
 
     def update_list(self, l):
         self._list = l
@@ -118,6 +117,14 @@ class BalanceToolButton(QToolButton, PieChartObject):
         QToolButton.paintEvent(self, event)
         PieChartObject.paintEvent(self, event)
 
+    def resizeEvent(self, e):
+        super().resizeEvent(e)
+        self._update_size()
+
+    def _update_size(self):
+        size = max(18, font_height(self))
+        self.R = QRect(6, 3, size, size)
+
 
 class LegendWidget(QWidget):
     size = 20

electrum/gui/qt/util.py

@@ -1262,8 +1262,10 @@ def char_width_in_lineedit() -> int:
     return max(9, char_width)
 
 
-def font_height() -> int:
+def font_height(widget: QWidget = None) -> int:
-    return QFontMetrics(QLabel().font()).height()
+    if widget is None:
+        widget = QLabel()
+    return QFontMetrics(widget.font()).height()
 
 
 def webopen(url: str):