An index that refers to a byte offset in a text run. This could the middle of a glyph.
A newtype struct for denoting the age of messages; prevents race conditions.
A unique ID for every stacking context.
If none of the bits outside this mask are set, the scroll root is a special scroll root. Note that we assume that the top 16 bits of the address space are unused on the platform.
The next ID that will be used for a special scroll root id.