What is wrong with the topic bar on OS X Yosemite?

Current guidance: This note describes a WebKit bug on OS X Yosemite. It is kept for historical reference; the demonstration attachment is retained in the source archive but not republished here.

Overview

Unfortunately, for users of OS X Yosemite, there exists a bug which results in the topic bar of Textual sometimes flickering or losing its position at the top of the window.

A bug report has been submitted to Apple and the WebKit project with the hope that it will be fixed.

Technical Information

For those interested, below is a copy of the bug report filed with Apple.

#18211024 — Embedded WebView flickers position: fixed elements during rapid scrolling events

Summary:
Using an embedded WebView inside of an application using the WebKit framework produces
flickering of position: fixed elements while rapidly scrolling the page. This behavior
did not exist until OS X Yosemite, Developer Preview #5 — it is believed to be a result
of: https://bugs.webkit.org/show_bug.cgi?id=135135

Steps to Reproduce:
1. Create an application with an embedded WebView that uses the version 1 of the WebKit
   provided WebView infrastructure. Not WKWebView. Normal WebView.
2. Load an HTML page with contains a position: fixed element
3. Scroll rapidly to produce flickering behavior

Expected Results:
The expected result is that the element stays statically fixed with no flickering during
scrolling events

Actual Results:
position: fixed HTML elements flicker during rapid scrolling events

Version:
OS X 10.10 (14A343f)

Notes:
Attached is an HTML document which produces this flickering as well as a video which shows
it in action.

Configuration:
Fresh install, base configuration

Attachments:
See attachment for video demonstrating the bug as well as example source code of an HTML
page which produces the bug. Use of this HTML page in a browser such as Safari will not
produce the behavior as Safari uses WKWebView. A 3rd party application such as Coda which
uses the responsible version of WebKit WebView can be used to demonstrate the behavior.

A copy of this bug has also been filed with the WebKit project as a critical bug with ticket #137851