Support setting the window dragging region with CSS

[paulrouget]

Glutin allows decoration-less windows. We can paint where the titlebar is supposed to be displayed by the OS, and then build our own titlebar (thinking of browser.html here).

But we need to be able to tell the OS what region of the UI is draggable.

The equivalent of -webkit-app-region: drag and -moz-window-dragging: drag;. See gecko implementation.

[SimonSapin]

Such CSS properties should be prefixed with -servo- (to avoid name collisions with possible future standard properties) and not exposed to content, possibly by checking style::parser::ParserContext::stylesheet_origin in the parsing code. Or perhaps we need another mechanism for "internal" properties.

[paulrouget]

Relevant gecko patches (haven't landed in m-c): https://bugzilla.mozilla.org/show_bug.cgi?id=1070710

[paulrouget]

This is what I tried to do:

• window.setMovableByWindowBackground_(YES)
• add NSViews where the window is not draggable
• these views are subclasses of NSView where mouseDownCanMoveWindow returns NO

But mouseDownCanMoveWindow is never called. I don't know why.

[paulrouget]

Failing code: paulrouget/glutin@5b922d1

[paulrouget]

This is not standard and not necessary anymore. Closing.