give Font a root widget

Fixes toolbar for tk.NoDefaultRoot() users

Author: richardsheridan

lib/matplotlib/backends/_backend_tk.py

@@ -543,7 +543,7 @@ def __init__(self, canvas, window, *, pack_toolbar=True):
                 if tooltip_text is not None:
                     ToolTip.createToolTip(button, tooltip_text)
 
-        self._label_font = tkinter.font.Font(size=10)
+        self._label_font = tkinter.font.Font(root=window, size=10)
 
         # This filler item ensures the toolbar is always at least two text
         # lines high. Otherwise the canvas gets redrawn as the mouse hovers