I had never seen that behavior until I investigated another forum question on the worksheet icon in the main toolbar:
Specifically the comment about ...
And as mentioned the button remains highlighted - looks extremely tacky.
Surprisingly, after using the worksheet drop-down list from the main toolbar, and leaving the icon in a "tacky" state, I noticed that my usually way of opening a connection (including the open worksheet on connect preference) from the Connections view no longer opened the worksheet. Unfortunately I have yet to understand the full details, but if your situation is similar then the following should help as a workaround:
1. Remove highlighting on the SQL icon in the main toolbar by clicking on it (but not adjacent drop-down arrow to its right!).
If the worksheet still will not open when opening a connection, try one or more of these (I feel a bit like a witch doctor ...):
2. Open Connection Selector UI by clicking on the SQL icon in main toolbar (not the drop-down arrow!), then close it.
3. Try some combination of closing/opening the Start Page. Not sure if matters in which state it ends up.
4. Disconnect Connections by right-clicking on the main Connections node in the Connections view.
Hope to get a bug filed for this, but it would be nice to nail down the test case and workaround first.
SQL Developer Team