Code is not highlighted in the Developer Console post test class execution

Root Cause

This issue occurs when the Store Only Aggregate Code Coverage option is enabled in the Apex Test Execution settings. When this option is checked, Salesforce stores only a summary of code coverage data — not the detailed, line-by-line coverage required to display highlighting in the Developer Console. Disabling this option allows Salesforce to store the granular coverage data needed for the highlighting feature.

How to Enable Code Coverage Highlighting

1. From Setup, search for and select Apex Test Execution in the Quick Find box.
2. Click the Options button.
3. Uncheck Store Only Aggregate Code Coverage if it is currently checked.
4. Click OK to save.
5. Re-run your test class.
6. Open the Apex class in the Developer Console — code coverage highlighting (blue for covered, red for uncovered) will now appear after the tests complete.