This is not about the 'tab control" design element in Composer ... it's about the tab bar at the top of Composer's "Editor" or "Design Editor" pane (that's what the Help docs call it).
1. Fix the tab display in Composer's "Editor" pane. Sometimes a tab's content will over/underwrite the tab navigation arrows:
2. Depending on the type of element selected, the "Pallet" shows or disappears. When that happens all the tabs jump around.
3. Depending on how a tab is selected, it's horizontal size changes and the other tabs move around. If a tab is "selected" by selecting it's element from the Navigator, the tab is "selected" but the text font on the tab is not BOLD. When that same tab is clicked, the tab text changes to BOLD font and the tab size changes. This causes the tab display to move around. This is most noticeable on design elements with very long names.
4. MOST IMPORTANT: all tabs for open design elements should always be visible -- the horizontal scrolling / "sliding" tab bar is frustrating to use. Horizontal scrolling is bad. I know browsers do it. It stinks there, too.
Multiple rows of tabs or some other mechanism? I can't offer any suggestions on a design paradigm, but there's gotta be something better.