On Apple machines Command + A should work to mark text in the changes editor.
|Duplicate||None||T86785 Excessive quantity of edits conflict in a busy project page|
|Open||cscott||T113004 Make it easy to fork, branch, and merge pages (or more)|
|Open||None||T72163 Edit conflicts (tracking)|
|Open||None||T139601 Better solutions to edit conflicts (#1)|
|Open||None||T161708 Mark all text should also work with Command + A|
If anyone want's to hack on this but needs help finding where to start, whom to speak with or similar – starting this week there is now a weekly Technical Advice IRC meeting open for all volunteer developers: https://www.mediawiki.org/wiki/Technical_Advice_IRC_Meeting