A case of occasional heap corruption was found when moving around a map with turf vis_contents.
BYOND Version:512.1434
Operating System:Windows 7 Ultimate 64-bit
Web Browser:Firefox 61.0
Applies to:Dream Seeker
Status: Resolved (512.1435)

This issue has been resolved.
Descriptive Problem Summary:

When under stress (particularly when the eye is moving and jumping rapidly), turf vis_contents will fail to appear at random.

Additionally, there have been apparent SERVER-SIDE hangs while working on the minimal test case, using Dream Maker's Compile and Run, where Dream Seeker is responsive but the world has stopped simulating. Dream Seeker has also crashed on quit at least once.

I'm still trying to diagnose some slowdowns (temporary hangs) and at least one crash with Dream Seeker in SS13. There are also reports from players of ALL turf vis_contents disappearing simultaneously and permanently (until reconnect), which has been reproduced but not in controlled circumstances. I'll post another report on those when I have more substance (though I do have a crash dump on-hand if you want it via PM or something).

Code Snippet (if applicable) to Reproduce Problem:
Minimal test project

Expected Results:

Flying around the map while "Toggle Gliding" is ON and pressing "Go Back" rapidly, and otherwise just sort of fiddling with the controls for several minutes, should show the vis_contents wherever they may be.

Actual Results:

Under stress, some vis_contents disappear:

Larger areas have been observed to go missing as well. The missing vis_contents appear again when they have gone more than three tiles out of view (except at the edge of the map, where "out of view" is calculated as if the view was clamped to the map edge).

If an area is missing, hitting LEFT (client/West) will cancel gliding so it does not go out of view too quickly for the failure to be observed.

Does the problem occur:
Every time? Or how often? Arbitrary, but often enough to appear in a test project.
In other games? Yes, in test project.
In other user accounts? Yes, reported by players.
On other computers? Yes, reported by players.

Lummox JR resolved issue with message:
A case of occasional heap corruption was found when moving around a map with turf vis_contents.