Hmm ... looks like it was a theme problem. I switched to the default "ambiance" theme and the problem went away. Sorry ... didn't think to try that first. I was using the "arc" theme when the switch to gtk3 happened.
The only other issue I've encountered since the switch is the inability to use the "Create New Subscription" dialog box. I can't type anything into the search box on either the "Search" or "RSS" tabs.
Other than that, everything seems good. I like the new look.
B02 might change things for the sidebar. I'm not sure though, since it looks the same to me, but I fiddled with the painting :)
B03 will "fix" the subscription dialog. The real textbox is actually under the fake one. I moved it above to you can click and type into it, but it appears I can't remove the border from it on SWT's GTK3 implementation :( You probably already noticed this ugly double border thing on the filter bubbles in Libarary/Files view.
Yes, I noticed the double-border thing but didn't think much of it. I did also discover that I could tab through the fields and buttons on the search dialog to get to the right place to enter my search term. I couldn't see what I was typing, but it got me what I needed. :)