Feedback from Julie Solon of Microsoft:

> These console functions share a per-process heap that is 64K. There is some overhead, the heap can get fragmented, and calls from multiple threads all affect how much is available for this buffer. 

> I am working to update the documentation for this function [WriteConsoleW] and other affected functions with information along these lines, and will post it within the next week or two.

I replied thanking her and asking for clarification:

When you say that the heap can get fragmented, is this true only when
there are concurrent calls to the console functions, or can it occur
even with single-threaded use? I'm trying to determine whether acquiring
a process-global lock while calling these functions would be sufficient
to ensure that the available heap space will not be unexpectedly low.
(This assumes that the functions not used outside the lock by other
libraries in the same process.)

ReadConsoleW seems also to be affected, incidentally.

I've asked for clarification about whether acquiring a process-global lock when using these functions ...
Julie