Horizontal divider position between tool windows is not saved when restarting

Summary:

This is a incidental bug I noticed relating to #5650

When you have two tool windows stacked vertically, Inkscape lets you position the horizontal divider but doesn't save its position when restarting.

Steps to reproduce:

• open Inkscape
• open layer dialogue (layer > Layers...)
• ensure layer dialogue is docked on the right
• open Path & Fill dialogue (ensure is also docked on the right)
• drag the tab for Path & Fill into the drop zone above layers to they stack
• reposition the horizontal divider until the Path & Fill dialogue is fully visible
• close Inkscape
• open Inkscape

What happened?

After opening Inkscape the second time the horizontal divider will have its position reset to the middle.

What should have happened?

The slider's position should still be where I left it before closing Inkscape.

Version info

Inkscape 1.1 (c68e22c387, 2021-05-23)

    GLib version:     2.68.2
    GTK version:      3.24.29
    glibmm version:   2.66.0
    gtkmm version:    3.24.3
    libxml2 version:  2.9.12
    libxslt version:  1.1.34
    Cairo version:    1.17.4
    Pango version:    1.48.5
    HarfBuzz version: 2.8.1
    Poppler version:  20.12.1

    OS version:       Windows 10 2009