A browser given a 390px viewport still lays the page out at the 980px desktop
fallback unless the document declares <meta name="viewport">. Measured in
Chromium at 390x844 with isMobile:true:
without the meta: window.innerWidth = 980 (body 964px)
with the meta: window.innerWidth = 390 (body 374px)
Nothing warns. The page renders, the probe reads its numbers, and the check
goes green having measured a viewport no phone has. A full set of mobile runs
was taken this way before the layout width was ever compared to the requested
one, so the conclusions drawn from them described the wrong viewport.
Three gates already read window.innerWidth and none compares it to the width it
asked for, so all three would pass at 980 while reporting 390. This asserts the
property that makes their numbers mean what they say.
Checked on the built artifact rather than the generator template: a template
containing the right string proves nothing about the HTML that shipped. No
server needed.
width=device-width is required specifically, not just the tag: a meta carrying
only initial-scale still lays out at 980, so matching the tag name alone would
leave the gate unable to fail on its own founding bug.
The 28 sheets under assets/posters/ are exempt by their stated shape - a fixed
paper canvas (.stage{width:1500px;height:2000px}) has no mobile layout to get
wrong. Keyed on that property, not the directory, so a poster keeps its
exemption if it moves and an app page in that folder does not inherit one.
Mutation-tested against the real tree: stripping the meta from a shipped page
fails it, and replacing it with an initial-scale-only tag fails it. 8 self-tests
cover both, plus the exemption and its inverse.
