This issue duplicates another issue.
220.127.116.119 RC - Side Tabs not correct in FF18.0.2
The side tabs example doesn't work correctly for right sided tabs.
You click on another tab then it's drawn incorrectly. FF 18.0.2 Win 7 Pro x64. Noticed also the RTL example doesn't work correctly collapsing and expanding the east and west regions using FF18.0.2 Win 7 Pro x64. Works fine in Chrome.sideTabsError.jpg
Thanks for posting, we caught this one in testing, ticket is EXTJSIV-8614 for your reference.
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