Star View Renders above Other Windows, Freezes When Min'd

[simulacrum]

Processor: Intel Core Duo T2500 (Dual Core, 2.00 GHz)
Memory: 2x512 MB sticks
OS: Ubuntu 9.04
Graphics: ATI Radeon Mobility X1400, latest Xorg driver
Celestia Version(s) Tested: 1.5.1 (default from Ubuntu repos), 1.5.0, 1.4.1

OpenGL Info (from Celestia Window):
Vendor: DRI R300 Project
Renderer: Mesa DRI R300 20060815 x86/MMX/SSE2 TCL
Version: 1.3 Mesa 7.4

When I load Celestia, when the view screen refreshes, it seems to render itself in a layer above everything else, like menus, options windows, even other applications. (Attached is a screenshot.) Also, any attempt to minimize the application causes the computer to freeze. The problems occur on all 3 versions that I tested (1.5.1, 1.5.0, 1.4.1). The problems also occur with both compiz and metacity.

I just installed Celestia on Ubuntu 9.04 yesterday, and I haven't experienced this problem before on previous versions of Ubuntu or Celestia, though I think it's been about a year since I've used it on my computer. I think I was using 8.04 at the time, but I'm not sure.

Seeing as the problem is persistent across all 3 versions of Celestia that I've tried, I think it's probably a software problem on my computer. I'm not sure exactly how my computer was configured when it all worked correctly, though.