Embedding in Tk example: Fix toolbar being clipped. (#15266)

jmehne · ImportanceOfBeingErnest · commit 9cea8eac7479 · 2019-09-15T14:58:05.000+02:00
Calling `pack` without options puts the widget at the end of the package list. Space is allocated to widgets sequentially. If a window is too small, there may be no space left for widgets. By packing the canvas last, we essentially give the toolbar and the quit button priority in the packing order. Fixes #13485.
diff --git a/examples/user_interfaces/embedding_in_tk_sgskip.py b/examples/user_interfaces/embedding_in_tk_sgskip.py
@@ -25,11 +25,9 @@
 
 canvas = FigureCanvasTkAgg(fig, master=root)  # A tk.DrawingArea.
 canvas.draw()
-canvas.get_tk_widget().pack(side=tkinter.TOP, fill=tkinter.BOTH, expand=1)
 
 toolbar = NavigationToolbar2Tk(canvas, root)
 toolbar.update()
-canvas.get_tk_widget().pack(side=tkinter.TOP, fill=tkinter.BOTH, expand=1)
 
 
 def on_key_press(event):
@@ -40,6 +38,12 @@ def on_key_press(event):
 canvas.mpl_connect("key_press_event", on_key_press)
 
 button = tkinter.Button(master=root, text="Quit", command=root.quit)
+
+# Packing order is important. Widgets are processed sequentially and if there
+# is no space left, because the window is too small, they are not displayed.
+# The canvas is rather flexible in its size, so we pack it last which makes
+# sure the UI controls are displayed as long as possible.
 button.pack(side=tkinter.BOTTOM)
+canvas.get_tk_widget().pack(side=tkinter.TOP, fill=tkinter.BOTH, expand=1)
 
 tkinter.mainloop()
