[FIX] widget html: rendering issue in mail composer on Firefox

The cleditor width does not include the margins. Setting 100% will make the editable area too large (104%) on Firefox (opw 611700).
This issue was already fixed in newer releases, from commit 9247c37d
Nevertheless, it introduced a new issue: while editing the form, the content of widgets html which were located in hidden notebook pages weren't displayed when displaying the page content.
opw-614448