^F on Plane of Water didn't show the magic portal to Astral. I spent
a lot of time looking at differences between current code and 3.4.3 to figure
out what had changed, then finally tried it on 3.4.3 and discovered that it
didn't work back then either. ^F marks all floor and ceiling traps as seen
and explicitly maps them, but Water has level.flags.hero_memory clear, making
the mapped trap disappear when it isn't in line of sight.
When fixing this, I changed magic mapping so that it shows furniture
(stairs, altars, fountains, &c) in preference to traps or objects, and known
traps in preference to objects, so hidden things can be spotted. (Once
they're in line of sight, the precedence reverses to normal, showing objects
on top of traps on top of floor.)
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