Thanks for the fix, it looks good to me. Could you add a comment in the code, since I don't think it's super obvious what pack_propagate() is achieving here. Something like # Don't resize the toolbar when mouse coordinates are shown or not shown.

Thanks for the fix, it looks good to me. Could you add a comment in the code, since I don't think it's super obvious what pack_propagate() is achieving here. Something like # Don't resize the toolbar when mouse coordinates are shown or not shown.

How about this:

         tk.Frame.__init__(self, master=window, borderwidth=2,
                           width=int(canvas.figure.bbox.width), height=50)
+        # Avoid message_label expanding the toolbar size, and in turn expanding
+        # the canvas size. When the user defines a small figure size(e.g. 2x2),
+        # this will cause:
+        # 1. Figure size that is bigger than the user's expectation.
+        # 2. When message_label is refreshed by mouse enter/leave, the canvas
+        #    size will also be changed.
         self.pack_propagate(False)

Basically yes. A bit clearer

... Without pack_propergate(False), when the user defines a small figure size(e.g. 2x2),

Did we also want to shrink the toolbar height, as noted in #28330 (comment)?

Did we also want to shrink the toolbar height, as noted in #28330 (comment)?

I'd be in favor of that.

Did we also want to shrink the toolbar height, as noted in #28330 (comment)?

Sorry, I used to think this display issue was simply due to the height of the toolbar.
However, after testing in both Windows and Linux environments, it seems to be related to DPI scaling.

(windows)

(linux)

When the scaling is set to 150% on windows, canvas is resized to 300 * 300, but the toolbar is resized to 300 * 50.
And on linux, the canvas is resized to 300 * 300 and toolbar is resized to 300 * 75.

This makes the appearance on Windows as expected, but not on Linux.

I'm not very clear on how rescaling works with TkAgg. Does anyone have experience with this?

I'm not an expert on this, but the relevant starting point in our code base is here:

I can't reproduce the toolbar issue on Linux regardless of DPI scaling setting. I'm going to merge as this does fix a long-standing problem.