Bootstrap/scss/forms/_floating-labels.scss

// We use mixins instead of placeholders as placeholders would combine the selectors when @extend-ed

.form-floating {
  position: relative;

  > .form-control,
  > .form-control-plaintext,
  > .form-select {
    height: $form-floating-height;
    min-height: $form-floating-height;
    line-height: $form-floating-line-height;
  }

  > label {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 2;
    height: 100%; // allow textareas
    padding: $form-floating-padding-y $form-floating-padding-x;
    overflow: hidden;
    text-align: start;
    text-overflow: ellipsis;
    white-space: nowrap;
    pointer-events: none;
    border: $input-border-width solid transparent; // Required for aligning label's text with the input as it affects inner box model
    transform-origin: 0 0;
    @include transition($form-floating-transition);
  }

  > .form-control,
  > .form-control-plaintext {
    padding: $form-floating-padding-y $form-floating-padding-x;

    &::placeholder {
      color: transparent;
    }

    &:focus::placeholder {
      color: $input-placeholder-color;
    }

    &:focus,
    &:not(:placeholder-shown) {
      @include form-floating-active-input-styles();
    }

    // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
    &:-webkit-autofill {
      @include form-floating-active-input-styles();
    }
  }

  > .form-select {
    @include form-floating-active-input-styles();
  }

  > .form-control:focus,
  > .form-control:not(:placeholder-shown),
  > .form-control-plaintext,
  > .form-select {
    ~ label {
      @include form-floating-active-label-styles();

      &::after {
        position: absolute;
        inset: $form-floating-padding-y ($form-floating-padding-x * .5);
        z-index: -1;
        height: $form-floating-label-height;
        content: "";
        background-color: $input-bg;
        @include border-radius($input-border-radius);
      }
    }
  }

  // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
  > .form-control:-webkit-autofill {
    ~ label {
      @include form-floating-active-label-styles();
    }
  }

  > .form-control-plaintext {
    ~ label {
      border-width: $input-border-width 0; // Required to properly position label text - as explained above
    }
  }

  > :disabled ~ label {
    color: $form-floating-label-disabled-color;

    &::after {
      background-color: $input-disabled-bg;
    }
  }
}

// Todo in v6: Consider combining this with the .form-control-plaintext rules to reduce some CSS?
.form-floating-always {
  &.form-floating,
  .form-floating {
    .form-control {
      @include form-floating-active-input-styles();

      &::placeholder,
      &:focus::placeholder {
        color: $input-placeholder-color;
      }
    }

    label {
      @include form-floating-active-label-styles();
    }
  }
}