mdb-ui-kit/scss/_forms.scss

form {
  // ensure enough room at the bottom of any form to display a one-line mdb-help
  margin-bottom: ($mdb-help-size-ratio * $font-size-base) * $line-height;

  // reverse the above for navbars (no help expected in a navbar form)
  .navbar & {
    margin-bottom: 0; // only adjust bottom so that pull-xs-right flexed margin-left: auto works

    .mdb-form-group {
      display: inline-block;
      padding-top: 0;
    }

    .btn {
      margin-bottom: 0;
    }
  }
}

// -----
// Inputs
//
// Reference http://www.google.com/design/spec/components/text-fields.html
// MDL implementation: http://www.getmdl.io/components/index.html#textfields-section
.form-control {
  // The border bottom should be static in all states, the decorator will be animated over this.
  &,
  &:focus,
  .mdb-form-group.is-focused & {
    border: 0;
    border-bottom: $input-btn-border-width solid $input-border-color;
  }

  @include mdb-disabled() {
    border-bottom: $input-btn-border-width dotted $input-border-color;
  }
}

// Focus underline animation =================================
.mdb-form-control-decorator {
  position: relative;
  top: (-1 * $input-btn-border-width); // move the top up enough to overlay the border from the input
  display: block;
  &::before,
  &::after {
    position: absolute;
    width: 0;
    height: $input-btn-border-width + 1;
    content: "";
    transition: 0.3s ease all;

    // focused state - it transitions the width of each (before and after) to 50% creating the center out effect
    .form-control:focus ~ &,
    .mdb-form-group.is-focused .form-control ~ & {
      width: 50%;
    }
  }
  &::before {
    left: 50%;
  }
  &::after {
    right: 50%;
  }
}

// End: Focus underline animation =================================

// Help blocks (not in v4)
//  position: absolute approach - uses no vertical space and there is no form jumping, but text wrapping - not so good.
//  FIXME: width/wrapping isn't automatic and overflows occur.  What are some solutions?
//
.mdb-help {
  position: absolute;
  display: none;
  font-size: 80%;
  font-weight: normal;
  @extend .text-muted;

  .mdb-form-group.is-focused & {
    display: block;
  }
}

// -----
// State coloring: default, success, info, warning, danger
//
@include mdb-selection-color();
@include mdb-form-color($mdb-label-color, $mdb-label-color-focus, $input-border-color);

.has-success {
  @include mdb-form-color($brand-success, $brand-success, $brand-success);
}

.has-info {
  @include mdb-form-color($brand-info, $brand-info, $brand-info);
}

.has-warning {
  @include mdb-form-color($brand-warning, $brand-warning, $brand-warning);
}

.has-danger {
  @include mdb-form-color($brand-danger, $brand-danger, $brand-danger);
}

// Reference http://www.google.com/design/spec/components/text-fields.html
// MDL implementation: http://www.getmdl.io/components/index.html#textfields-section
//.variations(unquote(" label"), color, $mdb-input-placeholder-color);  // default label color variations

// Whereas .form-group adds structure, mdb-form-group just needs to make sure we have enough padding for our labels to work.  That's the only purpose.
.mdb-form-group {
  position: relative;

  // -----
  // Labels
  //
  // Reference http://www.google.com/design/spec/components/text-fields.html
  // MDL implementation: http://www.getmdl.io/components/index.html#textfields-section
  [class^='mdb-label'],
  [class*=' mdb-label'] {
    position: absolute;
    pointer-events: none;
    transition: 0.3s ease all;

    // hint to browser for optimization
    &.mdb-label-floating {
      will-change: left, top, contents; // TODO: evaluate effectiveness - looking for community feedback
    }
  }

  // hide label-placeholders when the field is filled
  &.is-filled .mdb-label-placeholder {
    display: none;
  }

  // Optional class to make the text field inline collapsible/expandable (collapsed by default)
  //    This uses the BS collapse js to make the width expand.
  //    `width` class must also be on the element FIXME: do this with JS, it is a marker class and should be implicit because after all, we are an mdb-collapse-inline
  //    FIXME: js needs to do the focus on shown.bs.collapse event http://v4-alpha.getbootstrap.com/components/collapse/#events
  &.mdb-collapse-inline {
    padding: 0; // get rid of any padding as this is a width transition

    // Expandable Holder.
    .collapse {
      &.in {
        // This is an unfortunate hack. Animating between widths in percent (%)
        // in many browsers (Chrome, Firefox) only animates the inner visual style
        // of the input - the outer bounding box still 'jumps'.
        // Thus assume a sensible maximum, and animate to/from that value.
        max-width: 600px;
      }
    }

    .collapsing,
    .width:not(.collapse), // collapsing is removed and momentarily only width is present
    .collapse.in {
      display: inline-block;
    }

    .collapsing {
      @include material-animation-default();
    }
  }

  // default floating size/location with an mdb-form-group
  @include mdb-form-size-variant($font-size-base, $mdb-label-top-margin-base, $input-padding-y, $mdb-form-line-height, "mdb-form-group default");

  // sm floating size/location
  &.mdb-form-group-sm {
    @include mdb-form-size-variant($font-size-sm, $mdb-label-top-margin-sm, $input-padding-y-sm, $mdb-form-line-height-sm, "mdb-form-group sm");
  }

  // lg floating size/location
  &.mdb-form-group-lg {
    @include mdb-form-size-variant($font-size-lg, $mdb-label-top-margin-lg, $input-padding-y-lg, $mdb-form-line-height-sm, "mdb-form-group lg");
  }
}

// default floating size/location without a form-group (will skip form-group styles, and just render default sizing variation) - IMPORTANT for non-form-group spacing such as radio/checkbox/switch
@include mdb-form-size-variant($font-size-base, $mdb-label-top-margin-base, $input-padding-y, $mdb-form-line-height);

select {
  &,
  &.form-control {
    // Use vendor prefixes as `appearance` isn't part of the CSS spec. OSX doesn't obey the border-radius: 0 without this.
    -moz-appearance: none;
    -webkit-appearance: none;
  }
}

// Input files - hide actual input - requires specific markup in the sample.
//.mdb-form-group input[type=file] {
//  opacity: 0;
//  position: absolute;
//  top: 0;
//  right: 0;
//  bottom: 0;
//  left: 0;
//  width: 100%;
//  height: 100%;
//  z-index: 100;
//}

//
//
//.form-horizontal {
//
//  // Consistent vertical alignment of radios and checkboxes
//  .radio,
//  .checkbox,
//  .radio-inline,
//  .checkbox-inline {
//    padding-top: 0;
//  }
//
//  .radio {
//    margin-bottom: 10px;
//  }
//
//  label {
//    text-align: right;
//  }
//
//  label {
//    margin: 0;
//  }
//}