fix(19.7): RB-IN-04 document that a focused widget host mutes the plain keys

The custom-element-in-form rule that keeps Esc, / and ? out of a plugin
field type's closed shadow root also applies to a focusable widget that
is not an input. The header comment and isEditableTarget now state the
trade-off and why the rule is not narrowed to plugin field types.

admin/src/app/hotkeys.ts

@@ -23,6 +23,13 @@
 // dismissable layer (dialog, popover, dropdown, select, link popover) is
 // open: those close on Esc themselves without calling preventDefault, so the
 // DOM is checked for their data-dismissable-layer attribute instead.
+//
+// A focused custom element (a tag name with "-") inside a form counts as an
+// editable control, because a closed shadow root hides whether it holds an
+// input. The trade-off: a focusable widget that is not an input (a type:
+// widget chart or button strip) also mutes Esc, / and ? while it has focus;
+// they work again once focus leaves it. Narrowing the rule to plugin field
+// types would let Esc leave the form from a widget with an input inside.
 import { readonly, ref } from 'vue'
 import { t } from './i18n'
 
@@ -100,7 +107,8 @@ export function comboOf(event: KeyboardEvent): HotkeyCombo | null {
  * True when the target is inside an input, textarea, select or contenteditable
  * element, or is a custom element (a tag name with "-") inside a form: a
  * plugin field type whose closed shadow root hides the real input, where
- * composedPath() stops at the host.
+ * composedPath() stops at the host. Any such element counts, a focusable
+ * non-input widget included (see the header comment).
  */
 export function isEditableTarget(target: EventTarget | null): boolean {
   if (!target || typeof (target as Element).closest !== 'function') {