we used a custom 'data' property to pass around the items to add to the store, but this property is now used by ExtJS to store content for the component template also move to declarative style, saves lines
pveproxy with ExtJS 6 developpement mini howto
unpack the ExtJS 6 sources, and copy them to /usr/share/pve-manager/ext6
cd www/ext6/
make install
symlink the ext6 dir in pve-manager to the manager5 directory
cd /usr/share/pve-manager
ln -s PATH_TO_YOUR_GIT_REPO/www/manager6
access the PVE proxy with ExtJS 6
https://localhost:8006/?ext6=1
With the extra parameter ext6=1, pve-proxy will call the function PVE::ExtJSIndex6::get_index() which returns a HTML page, with all javascript symlinked from your git repository. Provided you included the javascript files in PVE/ExtJSIndex5.pm, after editing a file in the git repository, a simple refresh is enough to see your changes in the browser.