Editor: Fix tiny cursor in the empty editor

[ayshushus]

Fix #4249

add drawSelection() to CM.

Test in Nightly.

[ayshushus]

I spent a lot of time trying a native fix instead of a CM-drawn approach, this caused me to hard code a 10px offset positioning which I think is not scalable and in fact a potential footgun.

[flodolo]

Redirecting to Eemeli, since he knows more about CodeMirror than me.

[eemeli]

Looks reasonable. Could you also investigate if this does anything for #4240?

[ayshushus]

Could you also investigate if this does anything for 4240?

Yes. This has caused changes. They are to both the behavioural aspects you mentioned in #4240

1. cursor goes missing, press fn + arrow/Up/Cmd + arrow twice: on PR branch. This is currently the dancing pixel behavior in Pontoon.
2. cursor doesn't go to end of term, press fn + arrow/Up/Cmd + arrow twice on PR branch. This is currently the switching between the two ends of the placeholder in Pontoon.

[eemeli]

Could you describe the changes this PR makes to the behaviour of the cursor in RTL messages? I don't understand from your previous comment what the changes are, and whether they make the situation better or worse.