fix(material-experimental/mdc-form-field): use a CSS var for the floating label scale

This allows the value to be shared across CSS & JS.

Author: mmalerba

src/material-experimental/mdc-form-field/_form-field-density.scss

@@ -88,7 +88,8 @@
   .mat-mdc-text-field-wrapper.mdc-text-field--outlined .mdc-notched-outline--upgraded
   .mdc-floating-label--float-above {
     --mat-mdc-form-field-label-transform: translateY(
-            -#{mdc-textfield.get-outlined-label-position-y($height)}) scale(0.75);
+            -#{mdc-textfield.get-outlined-label-position-y($height)})
+            scale(var(--mat-mdc-form-field-floating-label-scale));
     transform: var(--mat-mdc-form-field-label-transform);
   }
 

src/material-experimental/mdc-form-field/_form-field-theme.scss

@@ -89,7 +89,8 @@
       // For the non-upgraded notch label (i.e. when rendered on the server), also
       // use the correct `body1` typography level.
       .mdc-floating-label--float-above {
-        font-size: mdc-typography.get-size(body1) * 0.75;
+        font-size: calc(
+            #{mdc-typography.get-size(body1)} * var(--mat-mdc-form-field-floating-label-scale));
       }
       .mdc-notched-outline--upgraded .mdc-floating-label--float-above {
         font-size: mdc-typography.get-size(body1);

src/material-experimental/mdc-form-field/form-field.scss

@@ -36,6 +36,9 @@
 // Host element of the form-field. It contains the mdc-text-field wrapper
 // and the subscript wrapper.
 .mat-mdc-form-field {
+  // The scale to use for the form-field's label when its in the floating position.
+  --mat-mdc-form-field-floating-label-scale: 0.75;
+
   display: inline-flex;
   // This container contains the text-field and the subscript. The subscript
   // should be displayed below the text-field. Hence the column direction.

src/material-experimental/mdc-form-field/form-field.ts

@@ -114,9 +114,6 @@ const FLOATING_LABEL_DEFAULT_DOCKED_TRANSFORM = `translateY(-50%)`;
  */
 const WRAPPER_HORIZONTAL_PADDING = 16;
 
-/** Amount by which to scale the label when the form field is focused. */
-const LABEL_SCALE = 0.75;
-
 /** Container for form controls that applies Material Design styling and behavior. */
 @Component({
   selector: 'mat-form-field',
@@ -285,6 +282,7 @@ export class MatFormField
   private _isFocused: boolean | null = null;
   private _explicitFormFieldControl: MatFormFieldControl<any>;
   private _needsOutlineLabelOffsetUpdateOnStable = false;
+  private _labelScale = 0;
 
   constructor(
     private _elementRef: ElementRef,
@@ -558,10 +556,17 @@ export class MatFormField
     if (!this._hasOutline() || !this._floatingLabel) {
       return;
     }
+    if (!this._labelScale) {
+      this._labelScale = Number(
+        getComputedStyle(this._elementRef.nativeElement).getPropertyValue(
+          '--mat-mdc-form-field-floating-label-scale',
+        ),
+      );
+    }
     // The outline notch should be based on the label width, but needs to respect the scaling
     // applied to the label if it actively floats. Since the label always floats when the notch
     // is open, the MDC text-field floating label scaling is respected in notch width calculation.
-    this._outlineNotchWidth = this._floatingLabel.getWidth() * LABEL_SCALE;
+    this._outlineNotchWidth = this._floatingLabel.getWidth() * this._labelScale;
   }
 
   /** Does any extra processing that is required when handling the hints. */