Selected cross-realm fixes (tldraw#6338)

A few small changes to make tldraw work in cross-realm based on
suggestions by @derekcicerone

Fixes tldraw#6337 and partially tldraw#5728 

### Change type

- [x] `bugfix`

### Release notes

- Fix a few code paths that didn't work for cross-realm situations.

Author: ds300

packages/editor/src/lib/editor/Editor.ts

@@ -3662,7 +3662,7 @@ export class Editor extends EventEmitter<TLEventMap> {
 	 * @public
 	 */
 	updateViewportScreenBounds(screenBounds: Box | HTMLElement, center = false): this {
-		if (screenBounds instanceof HTMLElement) {
+		if (!(screenBounds instanceof Box)) {
 			const rect = screenBounds.getBoundingClientRect()
 			screenBounds = new Box(
 				rect.left || rect.x,

packages/editor/src/lib/utils/dom.ts

@@ -18,7 +18,7 @@ import { debugFlags, pointerCaptureTrackingObject } from './debug-flags'
 
 /** @public */
 export function loopToHtmlElement(elm: Element): HTMLElement {
-	if (elm instanceof HTMLElement) return elm
+	if (elm.nodeType === Node.ELEMENT_NODE) return elm as HTMLElement
 	if (elm.parentElement) return loopToHtmlElement(elm.parentElement)
 	else throw Error('Could not find a parent element of an HTML type!')
 }

packages/tldraw/src/lib/ui/hooks/useKeyboardShortcuts.ts

@@ -33,16 +33,21 @@ export function useKeyboardShortcuts() {
 		if (!isFocused) return
 
 		const disposables = new Array<() => void>()
+		const container = editor.getContainer()
 
 		const hot = (keys: string, callback: (event: KeyboardEvent) => void) => {
-			hotkeys(keys, { element: document.body }, callback)
+			hotkeys(keys, { element: container.ownerDocument.body }, callback)
 			disposables.push(() => {
 				hotkeys.unbind(keys, callback)
 			})
 		}
 
 		const hotUp = (keys: string, callback: (event: KeyboardEvent) => void) => {
-			hotkeys(keys, { element: document.body, keyup: true, keydown: false }, callback)
+			hotkeys(
+				keys,
+				{ element: container.ownerDocument.body, keyup: true, keydown: false },
+				callback
+			)
 			disposables.push(() => {
 				hotkeys.unbind(keys, callback)
 			})