ace editor cursor behaves incorrectly

Ace can display only monospace fonts, The issue you describe can happen either if something changes font of the editor to non monospace, or assigns different fonts to different parts of the editor.

(There was a firefox addon on ubuntu wich was changing all fonts to non monospace)

I'm using mediawiki + chrome and encountered the same error.

Problem solved by using

.ace_editor, .ace_editor *{
font-family: "Monaco", "Menlo", "Ubuntu Mono", "Droid Sans Mono", "Consolas", monospace !important;
font-size: 12px !important;
font-weight: 400 !important;
letter-spacing: 0 !important;
}

in Mediawiki:Common.css

My same issue solved this way:

 #editor *{ font-family : monospace !important;font-size: 16px !important;direction:ltr !important;text-align:left !important;}