We were looking at the selection properties after we had mutated the DOM, and
trying to manipulate them based on some numbers cached from before mutating
the DOM, which could result in trying to set a negative index for the selection
offset. Instead, calculate all our values before we do any mutations.
Fixes#430
This is a massive refactor to port Squire to TypeScript, fix a bunch of small
bugs and modernise our tooling. The development was done on an internal
repository, so apologies to anyone following externally for the commit dump;
updates from here should come as real commits again.
Co-authored-by: Joe Woods <woods@fastmailteam.com>
