panel tabs not always visible even with enough room to show

[daxim]

1. In the right-hand area, activate the tab 'Directory'.
2. Shrink the right-hand area with the divider bar until only this tab is visible.
3. Select menu item Window → GoTo? Subs Window.
4. Expand the right-hand area until it is three to four times as wide.

What should happen:
All three tabs are displayed because they have enough room, the little triangles indicating that the tabs do not have enough room disappear.

What actually happens:
Only the tab 'Functions' is visible. There are triangles. The one pointing rightwards is disabled. The one pointing leftwards is enabled, clicking it multiple times restores the missing tabs.

[daxim]

Not fixed as of r18904.