fix(s2): Make gradient buttons have an animated transition

packages/@react-spectrum/s2/src/Button.tsx

@@ -138,24 +138,6 @@ const button = style<ButtonRenderProps & ButtonStyleProps & {isStaticColor: bool
       isDisabled: 'GrayText'
     }
   },
-  backgroundImage: {
-    variant: {
-      premium: {
-        default: linearGradient('96deg', ['fuchsia-900', 0], ['indigo-900', 66], ['blue-900', 100]),
-        isHovered: linearGradient('96deg', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100]),
-        isPressed: linearGradient('96deg', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100]),
-        isFocusVisible: linearGradient('96deg', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100])
-      },
-      genai: {
-        default: linearGradient('96deg', ['red-900', 0], ['magenta-900', 33], ['indigo-900', 100]),
-        isHovered: linearGradient('96deg', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100]),
-        isPressed: linearGradient('96deg', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100]),
-        isFocusVisible: linearGradient('96deg', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100])
-      }
-    },
-    isDisabled: 'none',
-    forcedColors: 'none'
-  },
   backgroundColor: {
     fillStyle: {
       fill: {
@@ -296,6 +278,42 @@ const button = style<ButtonRenderProps & ButtonStyleProps & {isStaticColor: bool
   disableTapHighlight: true
 }, getAllowedOverrides());
 
+// Put the gradient background on a separate element from the button to work around a Safari
+// bug where transitions of custom properties cause layout flickering if any properties use rems. 🤣
+// https://bugs.webkit.org/show_bug.cgi?id=285622
+const gradient = style({
+  position: 'absolute',
+  inset: 0,
+  zIndex: -1,
+  transition: 'default',
+  borderRadius: '[inherit]',
+  backgroundImage: {
+    variant: {
+      premium: {
+        default: linearGradient('to bottom right', ['fuchsia-900', 0], ['indigo-900', 66], ['blue-900', 100]),
+        isHovered: linearGradient('to bottom right', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100]),
+        isPressed: linearGradient('to bottom right', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100]),
+        isFocusVisible: linearGradient('to bottom right', ['fuchsia-1000', 0], ['indigo-1000', 66], ['blue-1000', 100])
+      },
+      genai: {
+        default: linearGradient('to bottom right', ['red-900', 0], ['magenta-900', 33], ['indigo-900', 100]),
+        isHovered: linearGradient('to bottom right', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100]),
+        isPressed: linearGradient('to bottom right', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100]),
+        isFocusVisible: linearGradient('to bottom right', ['red-1000', 0], ['magenta-1000', 33], ['indigo-1000', 100])
+      }
+    },
+    isDisabled: 'none',
+    forcedColors: 'none'
+  },
+  // Force gradient colors to remain static between light and dark theme.
+  colorScheme: {
+    variant: {
+      premium: 'light',
+      genai: 'light'
+    }
+  }
+});
+
 /**
  * Buttons allow users to perform an action.
  * They have multiple styles for various needs, and are ideal for calling attention to
@@ -350,65 +368,79 @@ export const Button = forwardRef(function Button(props: ButtonProps, ref: Focusa
         staticColor,
         isStaticColor: !!staticColor
       }, props.styles)}>
-      <Provider
-        values={[
-          [SkeletonContext, null],
-          [TextContext, {
-            styles: style({
-              paddingY: '--labelPadding',
-              order: 1,
-              opacity: {
-                default: 1,
-                isProgressVisible: 0
-              }
-            })({isProgressVisible}),
-            // @ts-ignore data-attributes allowed on all JSX elements, but adding to DOMProps has been problematic in the past
-            'data-rsp-slot': 'text'
-          }],
-          [IconContext, {
-            render: centerBaseline({slot: 'icon', styles: style({order: 0})}),
-            styles: style({
-              size: fontRelative(20),
-              marginStart: '--iconMargin',
-              flexShrink: 0,
-              opacity: {
-                default: 1,
-                isProgressVisible: 0
-              }
-            })({isProgressVisible})
-          }]
-        ]}>
-        {typeof props.children === 'string' ? <Text>{props.children}</Text> : props.children}
-        {isPending &&
-          <div
-            className={style({
-              position: 'absolute',
-              top: '[50%]',
-              left: '[50%]',
-              transform: 'translate(-50%, -50%)',
-              opacity: {
-                default: 0,
-                isProgressVisible: 1
-              }
-            })({isProgressVisible, isPending})}>
-            <ProgressCircle
-              isIndeterminate
-              aria-label={stringFormatter.format('button.pending')}
-              size="S"
-              staticColor={staticColor}
-              styles={style({
-                size: {
+      {(renderProps) => (<>
+        {variant === 'genai' || variant === 'premium' 
+          ? (
+            <span
+              className={gradient({
+                ...renderProps,
+                // Retain hover styles when an overlay is open.
+                isHovered: renderProps.isHovered || overlayTriggerState?.isOpen || false,
+                isDisabled: renderProps.isDisabled || isProgressVisible,
+                variant
+              })} />
+             )
+          : null}
+        <Provider
+          values={[
+            [SkeletonContext, null],
+            [TextContext, {
+              styles: style({
+                paddingY: '--labelPadding',
+                order: 1,
+                opacity: {
+                  default: 1,
+                  isProgressVisible: 0
+                }
+              })({isProgressVisible}),
+              // @ts-ignore data-attributes allowed on all JSX elements, but adding to DOMProps has been problematic in the past
+              'data-rsp-slot': 'text'
+            }],
+            [IconContext, {
+              render: centerBaseline({slot: 'icon', styles: style({order: 0})}),
+              styles: style({
+                size: fontRelative(20),
+                marginStart: '--iconMargin',
+                flexShrink: 0,
+                opacity: {
+                  default: 1,
+                  isProgressVisible: 0
+                }
+              })({isProgressVisible})
+            }]
+          ]}>
+          {typeof props.children === 'string' ? <Text>{props.children}</Text> : props.children}
+          {isPending &&
+            <div
+              className={style({
+                position: 'absolute',
+                top: '[50%]',
+                left: '[50%]',
+                transform: 'translate(-50%, -50%)',
+                opacity: {
+                  default: 0,
+                  isProgressVisible: 1
+                }
+              })({isProgressVisible, isPending})}>
+              <ProgressCircle
+                isIndeterminate
+                aria-label={stringFormatter.format('button.pending')}
+                size="S"
+                staticColor={staticColor}
+                styles={style({
                   size: {
-                    S: 14,
-                    M: 18,
-                    L: 20,
-                    XL: 24
+                    size: {
+                      S: 14,
+                      M: 18,
+                      L: 20,
+                      XL: 24
+                    }
                   }
-                }
-              })({size})} />
-          </div>
-        }
-      </Provider>
+                })({size})} />
+            </div>
+          }
+        </Provider>
+      </>)}
     </RACButton>
   );
 });

packages/@react-spectrum/s2/style/spectrum-theme.ts

@@ -10,7 +10,7 @@
  * governing permissions and limitations under the License.
  */
 
-import {ArbitraryValue, CSSValue, PropertyValueMap} from './types';
+import {ArbitraryValue, CSSProperties, CSSValue, PropertyValueMap} from './types';
 import {autoStaticColor, colorScale, colorToken, fontSizeToken, generateOverlayColorScale, getToken, simpleColorScale, weirdColorToken} from './tokens' with {type: 'macro'};
 import {Color, createArbitraryProperty, createColorProperty, createMappedProperty, createRenamedProperty, createSizingProperty, createTheme, parseArbitraryValue} from './style-macro';
 import type * as CSS from 'csstype';
@@ -111,8 +111,35 @@ export function colorMix(a: SpectrumColor, b: SpectrumColor, percent: number): `
   return `[color-mix(in srgb, ${parseColor(a)}, ${parseColor(b)} ${percent}%)]`;
 }
 
-export function linearGradient(angle: string, ...tokens: [SpectrumColor, number][]): string {
-  return `linear-gradient(${angle}, ${tokens.map(([color, stop]) => `${parseColor(color)} ${stop}%`)})`;
+interface LinearGradient {
+  type: 'linear-gradient',
+  angle: string,
+  stops: [SpectrumColor, number][]
+}
+
+export function linearGradient(this: MacroContext | void, angle: string, ...tokens: [SpectrumColor, number][]): [LinearGradient] {
+  // Generate @property rules for each gradient stop color. This allows the gradient to be animated.
+  let propertyDefinitions: string[] = [];
+  for (let i = 0; i < tokens.length; i++) {
+    propertyDefinitions.push(`@property --g${i} {
+  syntax: '<color>';
+  initial-value: #0000;
+  inherits: false;
+}`);
+  }
+
+  if (this && typeof this.addAsset === 'function') {
+    this.addAsset({
+      type: 'css',
+      content: propertyDefinitions.join('\n\n')
+    });
+  }
+
+  return [{
+    type: 'linear-gradient',
+    angle,
+    stops: tokens
+  }];
 }
 
 function generateSpacing<K extends number[]>(px: K): {[P in K[number]]: string} {
@@ -320,8 +347,10 @@ let gridTrackSize = (value: GridTrackSize) => {
 };
 
 const transitionProperty = {
-  default: 'color, background-color, border-color, text-decoration-color, fill, stroke, opacity, box-shadow, transform, translate, scale, rotate, filter, backdrop-filter',
-  colors: 'color, background-color, border-color, text-decoration-color, fill, stroke',
+  // var(--gp) is generated by the backgroundImage property when setting a gradient.
+  // It includes a list of all of the custom properties used for each color stop.
+  default: 'color, background-color, var(--gp), border-color, text-decoration-color, fill, stroke, opacity, box-shadow, transform, translate, scale, rotate, filter, backdrop-filter',
+  colors: 'color, background-color, var(--gp), border-color, text-decoration-color, fill, stroke',
   opacity: 'opacity',
   shadow: 'box-shadow',
   transform: 'transform, translate, scale, rotate',
@@ -802,7 +831,29 @@ export const style = createTheme({
     borderBottomEndRadius: createRenamedProperty('borderEndEndRadius', radius),
     forcedColorAdjust: ['auto', 'none'] as const,
     colorScheme: ['light', 'dark', 'light dark'] as const,
-    backgroundImage: createArbitraryProperty<string>(),
+    backgroundImage: createArbitraryProperty<string | [LinearGradient]>((value, property) => {
+      if (typeof value === 'string') {
+        return {[property]: value};
+      } else if (Array.isArray(value) && value[0]?.type === 'linear-gradient') {
+        let values: CSSProperties = {
+          [property]: `linear-gradient(${value[0].angle}, ${value[0].stops.map(([, stop], i) => `var(--g${i}) ${stop}%`)})`
+        };
+
+        // Create a CSS var for each color stop so the gradient can be transitioned.
+        // These are registered via @property in the `linearGradient` macro.
+        let properties: string[] = [];
+        value[0].stops.forEach(([color], i) => {
+          properties.push(`--g${i}`);
+          values[`--g${i}`] = parseColor(color);
+        });
+
+        // This is used by transition-property so we automatically transition all of the color stops.
+        values['--gp'] = properties.join(', ');
+        return values;
+      } else {
+        throw new Error('Unexpected backgroundImage value: ' + JSON.stringify(value));
+      }
+    }),
     // TODO: do we need separate x and y properties?
     backgroundPosition: ['bottom', 'center', 'left', 'left bottom', 'left top', 'right', 'right bottom', 'right top', 'top'] as const,
     backgroundSize: ['auto', 'cover', 'contain'] as const,

packages/@react-spectrum/s2/style/style-macro.ts

@@ -12,10 +12,10 @@
 
 import type {Condition, CSSProperties, CSSValue, CustomValue, PropertyFunction, PropertyValueDefinition, PropertyValueMap, RenderProps, ShorthandProperty, StyleFunction, StyleValue, Theme, ThemeProperties, Value} from './types';
 
-let defaultArbitraryProperty = <T extends Value>(value: T, property: string) => ({[property]: value} as CSSProperties);
-export function createArbitraryProperty<T extends Value>(fn: (value: T, property: string) => CSSProperties = defaultArbitraryProperty): PropertyFunction<T> {
+let defaultArbitraryProperty = <T>(value: T, property: string) => ({[property]: value} as CSSProperties);
+export function createArbitraryProperty<T>(fn: (value: T, property: string) => CSSProperties = defaultArbitraryProperty): PropertyFunction<T> {
   return (value, property) => {
-    let selector = Array.isArray(value) ? generateArbitraryValueSelector(value.map(v => String(v)).join('')) : generateArbitraryValueSelector(String(value));
+    let selector = Array.isArray(value) ? generateArbitraryValueSelector(value.map(v => JSON.stringify(v)).join('')) : generateArbitraryValueSelector(JSON.stringify(value));
     return {default: [fn(value, property), selector]};
   };
 }

packages/@react-spectrum/s2/style/types.ts

@@ -25,7 +25,7 @@ export type CSSProperties = CSS.Properties & {
   [k: CustomProperty]: CSSValue
 };
 
-export type PropertyFunction<T extends Value> = (value: T, property: string) => PropertyValueDefinition<[CSSProperties, string]>;
+export type PropertyFunction<T> = (value: T, property: string) => PropertyValueDefinition<[CSSProperties, string]>;
 
 export type ShorthandProperty<T> = (value: T) => {[name: string]: Value};