How to provide correct height and vertical scrolling of non-shinking flex child in column flex layout?

Question

In below column flex layout, the top and bottom paddings has been defined for .Layout-MainContent (orange part):

<div class="Layout">
  <div class="Layout-Header">
    <div class="Layout-Header-DummyLogo"></div>
  </div>
  <div class="Layout-MainContent">
    <div class="SignInForm">
      <div class="SignInForm-DummyHeading"></div>
      <div class="SignInForm-DummyInputField"></div>
      <div class="SignInForm-DummyInputField"></div>
      <div class="SignInForm-DummyInputField"></div>
    </div>
  </div>
</div>

html {
  height: 100%;
}

body {
  height: 100%;
  overflow: hidden;
}

.Layout {
  display: flex;
  flex-direction: column;
  height: 100%;
  background: #FFF9C4;
}


.Layout-Header {
  flex: 0 0 auto;
  padding: 12px 8px;
  background: #5C6BC0;
}

.Layout-Header-DummyLogo {
  width: 100px;
  height: 30px;
  background: #303F9F;
}

.Layout-MainContent {
  flex: 1 1 auto;
  overflow-y: auto;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 24px 0 36px;
  background: #FFCC80;
}

.SignInForm {
  flex: 0 0 auto;
  width: 320px;
  border-radius: 8px;
  padding: 32px 40px 20px;
  background: #42A5F5;
}

.SignInForm-DummyHeading {
  width: 100px;
  height: 30px;
  background: #1976D2;
}

.SignInForm-DummyInputField {
  margin-top: 24px;
  height: 30px;
  border-radius: 4px;
  background: #1976D2;
}

Fiddle

The first strange thing is bottom padding is not 36px as defined (update: looks like it's because of the JSFiddle console):

But this question topic is incorrect behavior on low viewports. I am expecting the vertical scrolling in .Layout-MainContent, but it's height must not become lower than natural (sum of paddings and children height). The real behavior is top and bottom paddings .Layout-MainContent are being completely ignored and also a part of .SignInForm has been truncated:

I know that flex: 1 1 auto; is contradicts to non-shinking requirement, but if we change to flex: 1 0 auto; the scrolling will completely disappear. How to keep both scrolling and minimal natural height if .Layout-MainContent