qt crash reporter: html.escape traceback to avoid formatting issues

fixes #6099

electrum/base_crash_reporter.py

@@ -121,9 +121,12 @@ class BaseCrashReporter(Logger):
             ['git', 'describe', '--always', '--dirty'], cwd=dir)
         return str(version, "utf8").strip()
 
+    def _get_traceback_str(self) -> str:
+        return "".join(traceback.format_exception(*self.exc_args))
+
     def get_report_string(self):
         info = self.get_additional_info()
-        info["traceback"] = "".join(traceback.format_exception(*self.exc_args))
+        info["traceback"] = self._get_traceback_str()
         return self.issue_template.format(**info)
 
     def get_user_description(self):

electrum/gui/qt/exception_window.py

@@ -22,6 +22,7 @@
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 import sys
+import html
 
 from PyQt5.QtCore import QObject
 import PyQt5.QtCore as QtCore
@@ -58,8 +59,6 @@ class Exception_Window(BaseCrashReporter, QWidget, MessageBoxMixin, Logger):
         main_box.addWidget(QLabel(BaseCrashReporter.REQUEST_HELP_MESSAGE))
 
         collapse_info = QPushButton(_("Show report contents"))
-        # FIXME if traceback contains special HTML characters, e.g. '<'
-        #       then formatting issues arise (due to rich_text=True)
         collapse_info.clicked.connect(
             lambda: self.msg_box(QMessageBox.NoIcon,
                                  self, _("Report contents"), self.get_report_string(),
@@ -139,6 +138,13 @@ class Exception_Window(BaseCrashReporter, QWidget, MessageBoxMixin, Logger):
     def get_wallet_type(self):
         return self.main_window.wallet.wallet_type
 
+    def _get_traceback_str(self) -> str:
+        # The msg_box that shows the report uses rich_text=True, so
+        # if traceback contains special HTML characters, e.g. '<',
+        # they need to be escaped to avoid formatting issues.
+        traceback_str = super()._get_traceback_str()
+        return html.escape(traceback_str)
+
 
 def _show_window(*args):
     if not Exception_Window._active_window: