ShowHeader and Sidebar problem in Visualforce page.

Hi,

I think if your Lightning is enabled for the org but when user is viewing in classic mode, custom visualforce pages aren't showing header or sidebar.

Adding standardStylesheets="true" can  solve the problem.

When you disable the inclusion of the Salesforce stylesheets, only your custom stylesheets affect the styling of the page. For the purposes of building up styles that partially or fully match the Salesforce look and feel, you might want to look at and use selected contents from the default stylesheets.

Salesforce stylesheets aren’t versioned, and the appearance and class names of components change without notice. Salesforce strongly recommends that you use Visualforce components that mimic the look-and-feel of Salesforce styles instead of directly referencing—and depending upon—Salesforce stylesheets.

Hope it may helps:

Thanks