region_highlight cancels styles set in zle_highlight

[Daniel Shahaf <d.s@daniel.shahaf.name>]

How do $zle_highlight and $region_highlight interact?  Which one takes
precedence if a certain byte range is covered by both?

(This came up in [1], which contains observations but no discussion.)

Empirically:

- if both $zle_highlight[region] and $region_highlight apply to a certain
  byte range, then that byte range is highlighted according to the
  former, only:

    bindkey -v
    zle_highlight=( region:underline )
    f() { region_highlight=("0 10 fg=red") }; zle -N f; bindkey -a ^T f
    echo foo bar<Esc><^T>v0
    [the 'echo foo bar' is red but not underlined]

- if both $zle_highlight[special] and $region_highlight apply to a certain
  byte, then both highlightings are applied "on top of" each other:

    zle_highlight=( special:underline )     
    f() { region_highlight=("0 10 fg=red") }; zle -N f; bindkey ^T f
    echo ^T bar «invoke f»
    [the ^T is red and underlined]
  
How does this work, in general?  When does zle merge the settings from
the two arrays, and when does one array override the other?

Thanks,

Daniel
(Background: zsh-syntax-highlighting wants the zle_highlight[region]
highlighting to be applied "on top of" the syntax highlighting [2]; the
workaround [3] works with current zle behaviour of "region_highlight
overrides zle_highlight[region]".)

[1] http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=29425
    Subject: region_highlight cancels styles set in zle_highlight
[2] https://github.com/zsh-users/zsh-syntax-highlighting/issues/165
[3] https://github.com/jimmijj/zsh-syntax-highlighting/commit/ece762e81798bc4448bf17f68a4792d1117dc032