Previously, topic edit diffs in the edit history modal were not
highlighted in the same way as content diffs because the highlighting
CSS rules were inside a .rendered_markdown block. So they affected the
content diffs, which are classed as such, but not the topic diffs.
This commit moves the highlight rules to a
.message_edit_history_content block inside the already existing
#message-edit-history block. .message_edit_history_content had
already existed in the edit history template message_edit_history.hbs,
and is assigned to both the content and topic diffs.
The ability to see topic edits in the edit history was added in
March 2019 in 38be5ea74394d2fd8586038de6ac447b4bbfbf67; the
highlighting worked at that time. It broke four mounths later in July
2019 in 38ffde37e5 when the highlight
rules were moved into a .rendered_markdown block after having been
global.
(As a further aside, .rendered_markdown was only added to the content
diffs in April 2019 in 5c36918c17.
.message_edit_history_content had been first added, to the content
diffs, in February 2019 in 7d42d7b4dbe6eb144a148135db50ad35efc01295.)
Aside from fixing topic edit diffs, this change is just more correct;
the highlight rules don't belong under .rendered_markdown, and they
don't need to be applied globally.
Sharif Naas