v5: Floating labels (#30449)

* v5: Promote floating labels example to component

- Adds new .form-floating
- Stubs out basics of a docs page
- Removes existing Example

* Update floating labels to support .form-select, make inputs and selects more consistent

- To do this, I made the .form-control and .form-select consistent in min-height vs height
- Removed some unused variables now
- Updated -color to be the -color because I don't know why this was any different before
- Update page to include some examples for layout, validation, and value
- Rewrite styles to not modify padding, but instead transform and opacity

* Streamline and bulletproof some things

- Apply some optimizations from code review
- Removed unecessary properties from the label
- Add some comments for what properties are required
- Move from fixed height for labels to height 100% so we can support textareas
- Improve docs a little bit, add ToC

* Move some values to variables, switch from scaling font-size to scale, update transforms

* Bring over changes from #30966 and add to them to tighten things up

* Delete the now unused example images

* Fix typo

* Allowlist the calc function

* Add transform-origin, update transform values

* Test out autofill fix

* Fix linter issue

* Mention it in the migration guide

* Bump bundlesize

* Add one more variable per review

* Shave .25rem off the height

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/forms/_floating-labels.scss

@@ -0,0 +1,85 @@
+// stylelint-disable selector-no-vendor-prefix
+
+.form-floating {
+  position: relative;
+
+  > .form-control,
+  > .form-select {
+    height: $form-floating-height;
+    padding: $form-floating-padding-y $form-floating-padding-x;
+  }
+
+  > label {
+    position: absolute;
+    top: 0;
+    left: 0;
+    height: 100%; // allow textareas
+    padding: $form-floating-padding-y $form-floating-padding-x;
+    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);
+  }
+
+  // stylelint-disable no-duplicate-selectors
+  > .form-control {
+    &::placeholder {
+      color: transparent;
+    }
+
+    &:focus,
+    &:not(:placeholder-shown) {
+      padding-top: $form-floating-input-padding-t;
+      padding-bottom: $form-floating-input-padding-b;
+    }
+    // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
+    &:-webkit-autofill {
+      padding-top: $form-floating-input-padding-t;
+      padding-bottom: $form-floating-input-padding-b;
+    }
+  }
+
+  > .form-select {
+    padding-top: $form-floating-input-padding-t;
+    padding-bottom: $form-floating-input-padding-b;
+  }
+
+  > .form-control:focus,
+  > .form-control:not(:placeholder-shown),
+  > .form-select {
+    ~ label {
+      opacity: $form-floating-label-opacity;
+      transform: $form-floating-label-transform;
+    }
+  }
+  // Duplicated because `:-webkit-autofill` invalidates other selectors when grouped
+  > .form-control:-webkit-autofill {
+    ~ label {
+      opacity: $form-floating-label-opacity;
+      transform: $form-floating-label-transform;
+    }
+  }
+  // stylelint-enable no-duplicate-selectors
+}
+
+
+//
+// Fallback for classic Edge
+//
+
+@supports (-ms-ime-align: auto) {
+  .form-floating {
+    display: flex;
+    flex-direction: column-reverse;
+  }
+  .form-floating > label {
+    position: static;
+    padding: 0;
+    margin-bottom: calc(#{$form-floating-padding-y} / 2); // stylelint-disable-line function-disallowed-list
+    border: 0;
+    @include transition(none);
+  }
+  .form-floating > .form-control::-ms-input-placeholder {
+    color: $input-placeholder-color;
+  }
+}

scss/forms/_form-select.scss

@@ -6,7 +6,7 @@
 .form-select {
   display: block;
   width: 100%;
-  height: $form-select-height;
+  min-height: $form-select-height;
   padding: $form-select-padding-y ($form-select-padding-x + $form-select-indicator-padding) $form-select-padding-y $form-select-padding-x;
   font-family: $form-select-font-family;
   @include font-size($form-select-font-size);