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kora bornkora born 

Trailhead explanation please

Can someone explain this clearly to a beginner overwhelmed with unecessary information about the classic Salesforce when all we are interested in is the new stuff ??

Note that both the sidebar and showHeader attribute have no effect in Lightning Experience, and that there’s no way to suppress the Lightning Experience header. Although the default value of showHeader is true, it has no effect in Lightning Experience.

Why on earth confuse us by tellings us to do something and saying it will not happen ?? Screenshots speak louder than words - old 'Confusious' saying.
SandhyaSandhya (Salesforce Developers) 
Hi,

As per my understanding 
showHeader is a Boolean value that specifies whether the Salesforce tab header is included in the page. If true, the tab header is displayed. If not specified, this value defaults to true.
Note: In Lightning Experience and the Salesforce app the value of this attribute is overridden, and is always false.

Also same is given in the guide below.

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_page.htm

 Best Regards,
Sandhya