open DevTools: CMD+OPT+I (on Mac) or F12 (on Windows)
hover your mouse over the listed Elements (in DevTools). You will have to expand div class="app-container" to drill down. Each hovered element will turn blue in the Obsidian window.
^ This will show you which Elements are causing that top-space.
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Now, in that same DevTools window, you can change/add Styles for each element (top-right corner).
In your case, the cause will probably be:
an element with margin-top or padding-top (causing that blank space). Set its property to margin-top: 0px; or padding-top: 0px;
an element displaying that “To download public links…” line. Set its property to display: none;