This request header contains the targeted layer's mode.
Server-side code is free to render different HTML for different modes.
For example, you might prefer to not render a site navigation for overlays.
Example
The user updates a fragment main within a drawer overlay.
Unpoly automatically includes the following request headers:
X-Up-Mode: drawer
X-Up-Target: main
The server chooses to render only the HTML required for the overlay.
It responds with the following HTTP:
Vary: X-Up-Mode
<main>...</main>
Note
Request headers that influenced a response should be listed in a Vary response header.
This tells Unpoly to partition its cache for that URL so that each
request header value gets a separate cache entries.
