Comment 99 for bug 195929

> I disagree here, the "transparent-bg-hint" should only be set if the engine is
> drawing directly on the canvas. This is not the case if Gecko fills in the
> background.

The engine can test if the style property is present. Whatever seems to be the most sane I will do.

> (In reply to comment #46)
> > I'll take the bug, and write a patch along the lines of comment #41, modulo the
> > naming which I will take from comment #42.
>
> I am not sure I understood everything from comment #41 :-)
>
> However, I do not see any advantage in registering the style property on
> GtkWidget (so that it exists on all widgets). AFAIK a style property is only
> needed for GtkEntry.

Apart from the naming part, comment 41 was telling that we shouldn't set the data on the widget if we aren't sure that we effectively can draw directly on the canvas. And that we shouldn't set this data only on entries or progress bars, but on every widget we can draw (while ensuring that the data isn't lying). If progressbar rendering is already "transparent-safe", good (but I hope that no theme relies on the progressbar being opaque).

As for registering the style property on GtkWidget, if it's absolutely sure that we won't need it on other widgets, and that it's not a problem if a theme tries to set the property when it's non-existent, no problem with me.