The top overview timeline doesn't resize when details sidebar appears, but the bottom timeline does resize. It's jarring, and I think the overview and main view should have the same behavior if possible.
<rdar://problem/16040319>
The top timeline is not expressed as a start and end time, it is expressed as a start time and seconds per pixel. So resizing the window will not grow or shrink the view. It is just revels more or less time. The bottom view is bound by a start and end time (from the selection), so it will grow and shrink. Showing the sidebar can be special cased to adjust the seconds per pixel on show/hide to give the illusion needed. But I'm not sure I agree with that.
I can see why that makes sense, it's just visually jarring if you are hoping to cross reference things on the sidebar against the timeline and then everything jumps.
I think we should revisit this, now that the Rendered Frames sidebar causes the timeline overview to jump position.
The Rendered Frames details sidebar will soon go away. It will be moved to the empty space in the navigation sidebar.
This is no longer an issue after https://bugs.webkit.org/show_bug.cgi?id=144346.