Dawn has this CSS in layout/theme.liquid:
body {
display: grid;
grid-template-rows: auto auto 1fr auto;
grid-template-columns: 100%;
min-height: 100%;
. . .
I believe the goal is to force footer to be at the bottom of the window if template content is not high enough to fill entire window.
The 1fr line supposed to be the <main id=“MainContent” ..> which will grow while other lines will fit contents.
However, you have an app EcomSend popups which creates an element above the and this breaks the layout.
On the screenshots: current site / display: grid; disabled / “offending” element removed.
If you plan on keeping the app, you may either remove display: grid; or, alternatively, change
grid-template-rows: from “auto auto 1fr auto” to “auto auto auto 1fr auto”.
Probably the better option is to have it like this – should be a bit more flexible then using grid 
body {
display: flex;
flex-direction: column;
}
main#MainContent {
flex-grow: 1;
}