Standardise widgets - always initialize y to 0 (#2982)

src/composables/widgets/useImagePreviewWidget.ts

@@ -235,6 +235,7 @@ class ImagePreviewWidget implements ICustomWidget {
   readonly options: IWidgetOptions<unknown>
   // Dummy value to satisfy type requirements
   value: string
+  y: number = 0
 
   constructor(name: string, options: IWidgetOptions<unknown>) {
     this.type = 'custom'

src/scripts/domWidget.ts

@@ -69,6 +69,7 @@ export class DOMWidgetImpl<T extends HTMLElement, V extends object | string>
   readonly element: T
   readonly options: DOMWidgetOptions<T, V>
   computedHeight?: number
+  y: number = 0
   callback?: (value: V) => void
 
   readonly id: string