Allow control of Slate's event handler execution in custom event handler API (#4299)

Co-authored-by: Georg Berecz <gberecz@palantir.com>
Co-authored-by: Claudéric Demers <clauderic.d@gmail.com>

packages/slate-react/src/components/editable.tsx

@@ -1191,13 +1191,19 @@ export const isEventHandled = <
   EventType extends React.SyntheticEvent<unknown, unknown>
 >(
   event: EventType,
-  handler?: (event: EventType) => void
+  handler?: (event: EventType) => void | boolean
 ) => {
   if (!handler) {
     return false
   }
+  // The custom event handler may return a boolean to specify whether the event
+  // shall be treated as being handled or not.
+  const shouldTreatEventAsHandled = handler(event)
+
+  if (shouldTreatEventAsHandled != null) {
+    return shouldTreatEventAsHandled
+  }
 
-  handler(event)
   return event.isDefaultPrevented() || event.isPropagationStopped()
 }
 
@@ -1207,12 +1213,19 @@ export const isEventHandled = <
 
 export const isDOMEventHandled = <E extends Event>(
   event: E,
-  handler?: (event: E) => void
+  handler?: (event: E) => void | boolean
 ) => {
   if (!handler) {
     return false
   }
 
-  handler(event)
+  // The custom event handler may return a boolean to specify whether the event
+  // shall be treated as being handled or not.
+  const shouldTreatEventAsHandled = handler(event)
+
+  if (shouldTreatEventAsHandled != null) {
+    return shouldTreatEventAsHandled
+  }
+
   return event.defaultPrevented
 }