Use a generic "widget with a selection handler" CSS class to make sure the handler is not cropped in all widgets

[oleq]

A follow–up of https://github.com/ckeditor/ckeditor5-block-quote/issues/28 where we brought some CSS to make sure that the selection handler of the table widget is not cropped when the table is a first-child of the block-quote or root editable (anything with overflow:hidden).

I think we should generalize the fix and instead of

.ck-editor__editable > .table:first-child,
.ck-editor__editable blockquote > .table:first-child {

we should refer to

.ck-editor__editable > .ck-widget.ck-widget_with-selection-handler:first-child,
.ck-editor__editable blockquote > .ck-widget.ck-widget_with-selection-handler:first-child {

when toWidget( ..., { hasSelectionHandler: true } ).

This will make the fix generic and work with all widgets we (and 3rd party developers) create in the future without constant updates to CSS.

In the future, if we develop different positions of the seleciton handler, we might need to make things dynamic, e.g. different styles (margin-top) when .ck-widget.ck-widget_with-selection-handler.ck-widget.ck-widget_selection-handler_top and (margin-botom) .ck-widget.ck-widget_with-selection-handler.ck-widget.ck-widget_selection-handler_bottom but I'm just thinking aloud.

cc @jodator @dkonopka

[oleq]

I just found out that we have the .ck-widget_selectable CSS class already added when hasSelectionHandler: true.

But it's totally wrong... all widgets are selectable (like the media widget, it has no handler but you can select it by simply clicking it). So we don't need to add another class, all we need to do is use this class but we need to rename it to something that makes sense. This class should indicate there's a selection handler, not that the widget it selectable (a very broad term).

[jodator]

Right now looks OK 👍

In the future, if we develop different positions of the seleciton handler, we might need to make things dynamic, e.g. different styles (

This looks promising so we shouldn't forget about it at least. Doing this right now without usages seems too early. But OTOH isn't a big change.