When you first open Safari and only have one page (one tab), the tab bar is not shown. When you open your second tab (e.g. by center-clicking a link to open it in a new tab), the tab bar then appears, which makes the content of the original page jump down a bit.
I expected to find an "Always show the tab bar" checkbox in Preferences, but apparently not.
Is there a way to force the tab bar to always appear, even if there's only one tab open?