New FigureCanvasTkAgg always keeps a black border - python-3.x

Question

I want to show some figures using tkinter, but new FigureCanvasTkAgg always keeps a black border. For example, I want to build two figures with red borders, but the new one has a black border, just like this:
enter image description here
But when the display window is not active, the black border disappear:
enter image description here
How to solve this problem? Thank you!
Here's the code:
import tkinter as tk
from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg
from matplotlib.figure import Figure
class display_window:
def __init__(self, width = 1024, height = 768):
self.figure_dict = {}
self.display_level = tk.Tk()
screen_width = self.display_level.winfo_screenwidth()
screen_height = self.display_level.winfo_screenheight()
init_position_x = int((screen_width - width) / 2)
init_position_y = int((screen_height - height) / 2)
position = str(width) + 'x' + str(height) + '+' + str(init_position_x) + '+' + str(init_position_y)
self.display_level.geometry(position)
self.x_offset = 120
self.y_offset = 10
self.figures_interval = 10
new_figure_button = tk.Button(self.display_level, text='new figure', command=self.new_figure_callback)
new_figure_button.place(x=5, y=5)
def new_figure_callback(self):
fig = Figure(figsize=(3, 2), dpi=100)
fig_plot = fig.add_subplot(111)
fig_plot.grid()
figure_canvas = FigureCanvasTkAgg(fig, self.display_level)
figure_widget = figure_canvas.get_tk_widget()
figure_widget.config(highlightthickness = 2, highlightbackground = "red", cursor='cross')
self.figure_dict[figure_widget] = {
"fig": fig,
"fig_plot": fig_plot,
"figure_canvas": figure_canvas,
}
self.arrange_figures(self.x_offset, self.y_offset, self.figures_interval)
def arrange_figures(self, x_offset, y_offset, figures_interval):
figures_area_width = self.display_level.winfo_width() - x_offset - figures_interval
figures_area_height = self.display_level.winfo_height() - y_offset - figures_interval
figure_count = len(self.figure_dict)
figure_width = figures_area_width
figure_height = (figures_area_height - figures_interval * (figure_count - 1)) / figure_count
for i, it in enumerate(self.figure_dict.keys()):
it.config(height = figure_height, width = figure_width)
it.place(x = x_offset, y = y_offset + i * (figure_height + figures_interval))
if __name__ == '__main__':
display_window()
tk.mainloop()
I want all the figures' borders display as in the config function.