qt history: fix slowness

arghhhhh finalllllllllllly figured it out...

electrum/gui/qt/history_list.py

@@ -96,9 +96,8 @@ class HistoryModel(QAbstractItemModel, PrintError):
         return self.createIndex(row, column)
 
     def data(self, index: QModelIndex, role: Qt.ItemDataRole):
-        # requires PyQt5 5.11
-        # indexIsValid = QAbstractItemModel.CheckIndexOptions(QAbstractItemModel.CheckIndexOption.IndexIsValid.value)
-        # assert self.checkIndex(index, indexIsValid)
+        # note: this method is performance-critical.
+        # it is called a lot, and so must run extremely fast.
         assert index.isValid()
         col = index.column()
         tx_item = self.transactions.value_from_pos(index.row())

electrum/gui/qt/util.py

@@ -444,6 +444,13 @@ class MyTreeView(QTreeView):
         self.setRootIsDecorated(False)  # remove left margin
         self.toolbar_shown = False
 
+        # When figuring out the size of columns, Qt by default looks at
+        # the first 1000 rows (at least if resize mode is QHeaderView.ResizeToContents).
+        # This would be REALLY SLOW, and it's not perfect anyway.
+        # So to speed the UI up considerably, set it to
+        # only look at as many rows as currently visible.
+        self.header().setResizeContentsPrecision(0)
+
     def set_editability(self, items):
         for idx, i in enumerate(items):
             i.setEditable(idx in self.editable_columns)