Fix right-click on downtime activities on macOS.

On macOS with a single-button mouse (e.g. a laptop trackpad) it's common to
trigger a right-click with ctrl+click.

In Chrome, this triggers both a `contextmenu` event and a regular `click`
event. In the context of downtime actions, this meant that we were
deselecting an action in the `contextmenu` handler but then immediately
re-selecting it again in the `click` handler.

This commit works around the problem by stopping the event from propagating
further. This fixes the bug, but also stops Foundry's default `contextmenu`
handler from firing and preventing the browser context menu from appearing,
so we also have prevent the event's default behaviour from firing.

module/applications/dialogs/downtime.mjs

@@ -119,6 +119,18 @@ export default class DhpDowntime extends HandlebarsApplicationMixin(ApplicationV
             : 0;
 
         this.render();
+
+        // On macOS with a single-button mouse (e.g. a laptop trackpad),
+        // right-click is triggered with ctrl+click, which triggers both a
+        // `contextmenu` event and a regular click event. We need to stop
+        // event propagation to prevent the click event from triggering the
+        // `selectMove` function and undoing the change we just made.
+        event.stopPropagation();
+
+        // Having stopped propagation, we're no longer subject to Foundry's
+        // default `contextmenu` handler, so we also have to prevent the
+        // default behaviour to prevent a context menu from appearing.
+        event.preventDefault();
     }
 
     static async takeDowntime() {