The sign-in form was 778px below the fold
The problem
Mobile users could not find the sign-in form. It rendered far below the viewport and overflowed the screen horizontally, so the page looked broken on a phone.
What we did
Measured the real layout in a headless browser at phone width instead of relying on a responsive preview, then fixed the container and the overflow at the source.
Result
Form reaches the first screen on a 390px viewport with no horizontal overflow.
How it was measured
The layout was measured in a headless browser at 390 by 844 pixels, reading the actual bounding box of the form rather than looking at a responsive preview. That distinction is the whole case: the preview showed a form, and the real viewport put it 778 pixels down the page with the container overflowing sideways. The same measurement was re-run after the fix, which is where the second number comes from.