I can't see nexus when i open browser or some app. But, when i drop my pointer at bottom, nexus will show it self. But i wanna when i open some browser or app, nexus can show it self, like toolbar in windows. How can do that?

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Can't be certain without seeing how you have the dock configured - however check the following:

In dock properties -> Position tab -> check that the tick box for 'Prevent maximised windows from overlapping the dock' is selected.

Also in dock properties -> Behaviour tab -> make sure Auto-hide tick boxes are unselected.