This patch should not affect existing behavior, but introduces a variable that would allow (e.g.) an external package to do so. One way for an external package to terminate an ongoing preview generation is via (TeX-kill-job). This approach is unreliable: if the timing is just right, then the kill signal arrives too late. This patch gives a more reliable way, based on a buffer-local flag. (The motivation is that one might wish to avoid generating previews when subsequent edits to the buffer make them irrelevant or misplaced.) Thanks, best, Paul