code: function(tabId) {
          var entry = tabContexts.get(tabId);
          if ( entry !== undefined ) {
              return entry;
          }
          // https://github.com/chrisaljoudi/uBlock/issues/1025
          // Google Hangout popup opens without a root frame. So for now we will
          // just discard that best-guess root frame if it is too far in the
          // future, at which point it ceases to be a "best guess".
          if ( mostRecentRootDocURL !== '' && mostRecentRootDocURLTimestamp + 500 < Date.now() ) {
              mostRecentRootDocURL = '';
          }
          // https://github.com/chrisaljoudi/uBlock/issues/1001
          // Not a behind-the-scene request, yet no page store found for the
          // tab id: we will thus bind the last-seen root document to the
          // unbound tab. It's a guess, but better than ending up filtering
          // nothing at all.
          if ( mostRecentRootDocURL !== '' ) {
              return push(tabId, mostRecentRootDocURL);
          }
          // If all else fail at finding a page store, re-categorize the
          // request as behind-the-scene. At least this ensures that ultimately
          // the user can still inspect/filter those net requests which were
          // about to fall through the cracks.
          // Example: Chromium + case #12 at
          //          http://raymondhill.net/ublock/popup.html
          return tabContexts.get(vAPI.noTabId);
      }
docs: Find a tab context for a specific tab. If none is found, attempt to fix this. When all fail, the behind-the-scene context is returned.