The undo stack needs to store regions [$15]

[erbridge]

Migrated from limetext/lime#211.

A view's associated regions need to be saved away in the undo-stack (or in edit objects perhaps).

For example, if I first load a file, then add a couple of regions, then delete the contents of the file. All regions will now have collapsed and merged into a single Region{0,0}, which is expected behavior.

Then I hit undo. I would now expect the regions to be what they were before I deleted the buffer contents, however what they will be instead is a single region covering the whole buffer.

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