Another run at the problem 57516164de was meant to solve. The
new solution just gives up when a `drawRect:' method is called
in the wrong thread, which might create refresh glitches but
avoids a deadlock in the case that the Racket thread is blocked
on the update happening in the foreign thread.
On Cocoa, a view's `drawRect:' method can be called from a
heartbeat thread that animates controls. Such a call happens
rarely for a `canvas%' or other class where `drawRect:'
is overridden, but since it can happen, ensure that the
callback runs on the Racket thread.
I started from tabs that are not on the beginning of lines, and in
several places I did further cleanings.
If you're worried about knowing who wrote some code, for example, if you
get to this commit in "git blame", then note that you can use the "-w"
flag in many git commands to ignore whitespaces. For example, to see
per-line authors, use "git blame -w <file>". Another example: to see
the (*much* smaller) non-whitespace changes in this (or any other)
commit, use "git log -p -w -1 <sha1>".
The target of the change is actually `hack-argument-replacement' in
`mred/private/coco/queue'. The replaced method can get called in
a different OS-level thread than the main thread.
Closes PR 12086
