Font selection box not scrollable vertically the first time it is opened

Migrated from: https://bugs.launchpad.net/inkscape/+bug/986936

Steps to reproduce:

• open Inkscape
• click on the text tool and click somewhere in the document to create text
• open the font dialog
• try to scroll vertically (with the mouse wheel or with a scrollbar)

What happened?

Scrolling with the mouse wheel does not work, and there is no vertical scrollbar.

Simple workaround: closing the list and re-opening it.

What should have happened?

Scrolling should work and a vertical scrollbar should be displayed.

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.1-dev (74036898e9, 2020-05-07) Pango version: 1.44.7. Compiled from INKSCAPE_1_0_ALPHA2-1689-g74036898e9.
• Operating System: Debian testing/unstable

Relevant comment from the previous report:

Initial investigation suggests that the problem may be occurring because the dropdown list displays while the list itself is being populated and cells rendered by calls to src/libnrtype/font- lister.cpp:font_lister_cell_data_funct. In particular I am suspicious about the call to gtk_rc_parse_string in src/widgets/text- toolbar.cpp:sp_text_toolbox_prep that occurs before the calls to font_lister_cell_data_funct have completed. (Artificially slowing down the latter causes the incomplete list to be clipped to a smaller height.) Yet we couldn't find an easy fix.