Still seeing this behavior under version 3.1 (28).
I can type into one tab, then switch, type some more, then switch back and type more when I hit undo it erases the changes in both tabs in the order that they were made, so the most recent changes in the current tab then it erases from the other tab and then back to the current tab, etc. This caused me to accidentally delete code in other tabs before I realized what it was doing and I'd wonder where my code went...
I can type into one tab, then switch, type some more, then switch back and type more when I hit undo it erases the changes in both tabs in the order that they were made, so the most recent changes in the current tab then it erases from the other tab and then back to the current tab, etc. This caused me to accidentally delete code in other tabs before I realized what it was doing and I'd wonder where my code went...