Now that Frame knows the visible viewport rect, it can easily ignore
repaint requests from e.g <blink> elements that are not currently
scrolled into view. :^)
When the visible viewport rect changes, we walk the layout tree and
check where each LayoutImage is in relation to the viewport rect.
Images outside have their bitmaps marked as volatile.
Note that the bitmaps are managed by ImageDecoder objects. If a bitmap
is purged by the kernel while volatile, we construct a new ImageDecoder
next time we need pixels for the image.
Just in time for Serenity's 1st birthday, here is the <blink> element!
This patch adds a bunch of different mechanisms to enable partial
repaints of the layout tree (LayoutNode::set_needs_display()))
It also adds LayoutNode::is_visible(), which can be toggled to prevent
a LayoutNode from rendering anything (it still takes up space though.)
Each HtmlView now has a main_frame(), which represents the main frame
of the web page. Frame inherits from TreeNode<Frame>, which will allow
us to someday implement a Frame tree (to support the <frame> element.)
This also fixes another bug with inline wrappers. Namely,
we should only add inline wrappers if a block node has
both non-block (inline or text) and block children.
