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Emmanuele Bassi authored
Firefox does a bunch of interesting things with GTK. If the top-level GtkWindow does not have a "csd" style class associated, Firefox will happily draw the contents of the container used to render HTML and XUL directly on the top level's GdkWindow; on the other hand, if a "csd" style class is found, the MozContainer will create a new child window, and draw on it. Then, Firefox will proceed to disable double buffering on both the top-level window and the MozContainer (unless they are backed by the same GdkWindow, in which case only the top-level will be single-buffered) *and* it will add a GDK_EXPOSURE_MASK flag to the MozContainer events for good measure (even if this is only needed for GTK+ 2.x). After landing the GdkDrawingContext API in GdkWindow, GTK enabled automatic double buffering on all top-level windows backed by a native surface, ad most users of single buffering rely on child widgets instead of top-levels, and we'd still like to have the same double buffering behaviour for all top-levels on all backends. Obviously, with Firefox disabling double buffering on the top-level window, the change broke their drawing mechanism. Ideally, Firefox could be fixed to not disable double buffering on the top-level window when MozContainer has a separate GdkWindow — i.e. the CSD case — but since we did introduce a slight change of behaviour in fringe users of the GTK+ API, let's keep backwards compatibility with the old code for a little while longer, and create an intermediate Cairo context unbound from the GdkDrawingContext, like we used to do until GTK+ 3.20.
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