SUMMARY
I couldn't login to userbase.kde.org, there was no login button/link. Then I made the window wider,and "Login with MyKDE" appeared (along with a language selector and Search... box).

This happens with Firefox and chromium-browser. I use a half-width browser on a full HD screen, thus it's normally 960 pixels wide.

The top-most element that vanishes is I think

  <div class="navbar-collapse collapse show" id="navbarPersonal" style="">

I notice in the development inspector the CSS style:

  @media (min-width: 992px)
  .navbar-expand-lg .navbar-toggler {
      display: none;
  }

so I think it vanishing is intentional. There's a button to show navigation,
  <button class="navbar-toggler webfonts-changed" 

but it's not visible until the window narrows below 767 pixels, at which point the left-hand navigation column also disappears and you see a strange '=/…' glyph that when clicked shows both the left-hand navigation and the navbar-personal, although the latter's elements display on top of the page content.

STEPS TO REPRODUCE
1. Visit https://userbase.kde.org/Quick_Start
2. Resize your browser window narrow and wider.

OBSERVED RESULT
Below 992 pixels wide, the language selector, Login with MyKDE and Search... box vanish with no way to show them.
Below 767 pixels wide, a toggle button appears that displays them (poorly) along with the left-hand navigation.
See the several attachments.

EXPECTED RESULT
Leave these items visible all the way down to 767 pixels. Or make the existing toggle appear at wider resolutions. Or have a separate toggle for these items. Or...

SOFTWARE/OS VERSIONS
KDE version: n/a
Chromium 88.0.4324.182 Fedora Project
Mozilla Firefox 88.0a1 (nightly)