This refers to the automatic highlighting according to code coverage that is activated via the "Analyze->Squish Coco..." dialog (if the Coco plugin is activated).

After setting the CoverageBrowser path and the .csmes file in the dialog, CoverageBrowser is started and highlighting works as expected. But when Qt Creator is then closed and restarted, CoverageBrowser is not started and there is no highlighting.

(Workaround: Open "Analyze->Squish Coco..." and press the "Open" button.)