Desktop: Performance: Reduces re-rendering caused in markupToHtml

[ken1kob]

This PR is one of PRs resolving issue #6386 and has two specs.

Spec 1

This PR suppresses redundant re-rendering of NoteEditor when a note is rendered by Markdown Editor/Viewer.

In CodeMirror.tsx, a note's markup text is converted into an Html text and stored into renderedBody variable, and simultaneously its hash is calculated and stored into renderedBodyContentKey variable. Since the two variables are useState variables, one conversion triggers re-rendering twice, one of which is redundant.

By changing renderedBodyContentKey into a useRef variable, re-rendering is triggered only once.

Spec 2

Additionally, the time of the conversion when a new note is loaded is changed. It was executed a bit later in a timeout handler. But now, it becomes being executed right away.

[ken1kob]

This commit was replaced to be mergeable with v2.9.17.

[laurent22]

In CodeMirror.tsx, a note's markup text is converted into an Html text and stored into renderedBody variable, and simultaneously its hash is calculated and stored into renderedBodyContentKey variable.

Since the key is derived from the body, can't we just use useMemo to handle the caching without having to use a ref?

[ken1kob]

Since the key is derived from the body, can't we just use useMemo to handle the caching without having to use a ref?

Since the update and reference of renderedBodyContentKey useRef variable are already in a useEffect function and are tightly coupled with a side effect, it cannot be replaced with useMemo.

[laurent22]

See https://github.com/laurent22/joplin/pull/6446#issuecomment-1817817260