Here is another approach a rival Media player uses regarding locked tabs: it has no lock icon of any kind... ever... so no wasted space. It works is like this:
- To close unlocked tabs: MMB just like MC, or RMB the tab and choose "close tab" from the menu (fyi... this player does not use tab close buttons at all- MMB to close is essentially standard)
- To close locked tabs: MMB is ignored - no warning, no info dialog pop-up... nothing. You must RMB and select close tab from the menu to close locked tabs, which protects against accidental tab closure. You can also unlock the tab first and then close via MMB.
After one closes or attempts to close locked and unlocked tabs a few times, the process becomes straightforward - and no need to allocate space to the side or below the tab text, or need to create new lock icons. Your suggestion to underline Tab text to indicate "locked" does not use any extra horizontal space, but underlined text means "link" to most users (e.g., all the underlined text links on the right side of the tabbar, which are links).
We both agree that minimizing the amount of space tabs take up is important, but maybe not on the best approach for how to do it. Since Matt recently created a new option to hide or show the tab close buttons, perhaps he could do the same with the tab lock icons? That is, if a user chooses to hide the tab lock icon, MMB on a locked tab produces the same warning we get right now "The tab is locked. Are you sure you want to close this tab?". If the tab is unlocked, then MMB simply closes the tab.