hooking spinner to GtkSidebar page transition

[Daiki Ueno <ueno gnu org>]

Hello,

I'm trying to replace my application's own sidebar implementation (based
on GtkListBox/GtkStack) with GtkSidebar, to take advantage of the
standard CSS theme[1].  Currently I have a small obstacle to this
migration.

To reduce the startup time, the application delays populating a page
widget in GtkStack until the first time it is displayed, and it
sometimes takes time.  So, until the page widget becomes ready, the
application shows a placeholder page with a spinner.

The actual interaction is:

1. a user clicks "Punctuation" tab for the first time
  2. the application starts loading Unicode punctuation characters in
     a background thread; if it takes long, a placeholder page is shown
    3. the "Punctuation" page is shown

So, my question is, is there any easy way to inject the phase 2 into the
page transition of GtkSidebar?

Thanks,

Footnotes: 
[1]  https://bugzilla.gnome.org/show_bug.cgi?id=740261

--
Daiki Ueno