We have several Tabs overridden by Visualforce pages. Ohter Visualforce pages are reached via a Link.
Until Summer 18 as default all these pages were rendered with Header and Sidebar. So on most of the pages, I did not use the attributes showHeader and sidebar on the apex:page
Now starting today, the Headers and Sidebars are missing. I've already tried to set some attributes explicitly on the apex:page
<apex:page standardController="MyCustomObject__c"extensions="MyApexExtension" showHeader="true" sidebar="true" standardStylesheets="true">
Also I made sure, that I'm not in Lightning-Experience-Mode, by switching to LEX and back to Classic using the Switcher under MyName ==> Switch to...
So whatever I set for these attributes and after double-checking that the user is in Classic-Mode, the pages are rendered always without Header and Sidebar.
Is there any way to get the pages redered with header and sidebar again?
Update
I found a very desperate workaround to get rid of the sidebar and header amputation by Summer 18 by completely turning LEX off. This global kill-switch is NOT what I want and can IMOH only be an intermediate solution.
So in parts @sfdcfox is right: the system thinks that I'm in LEX. Only that I'm not and LEX is just not disabled. For me, it looks like a bug of Summer 18. I'm not sure if support would classify this as an developer issue which would prevent me for logging a ticket for this possible bug.