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→Reflow: Update links and documentation about reflow roots
The reflow generally starts from the root of the frame tree, though some other
types of frame can act as "reflow roots " and start a reflow from them(nsTextControlFrame is one example; see the[https://searchfox.org/mozilla-central/search?q=symbol:E_%3CT_nsFrameState%3E_NS_FRAME_REFLOW_ROOT&redirect=true NS_FRAME_REFLOW_ROOT] frame state bit).
Reflow roots must obey the invariant that a change inside one of their
descendants never changes their rect or overflow areas (though currently
To do this, we maintain two bits on frames:
[httphttps://dxr.mozillasearchfox.org/mozilla-central/search?q=varsymbol:E_%3ANS_FRAME_IS_DIRTY&case=true3CT_nsFrameState%3E_NS_FRAME_IS_DIRTY&redirect=true NS_FRAME_IS_DIRTY]
indicates that a frame and all of its descendants require reflow.
[httphttps://dxr.mozillasearchfox.org/mozilla-central/search?q=varsymbol:E_%3ANS_FRAME_HAS_DIRTY_CHILDREN&case=true3CT_nsFrameState%3E_NS_FRAME_HAS_DIRTY_CHILDREN&redirect=true NS_FRAME_HAS_DIRTY_CHILDREN]
indicates that a frame has a descendant that
is dirty or has had a descendant removed (i.e., that it has a child that
has NS_FRAME_IS_DIRTY or NS_FRAME_HAS_DIRTY_CHILDREN or it had a child
removed). These bits allow coalescing of multiple updates; this
coalescing is done in nsPresShellPresShell, which tracks the set of reflow roots
that require reflow. The bits are set during calls to
The layout algorithms used by many of the frame classes are those
