BYOND Version:511
Operating System:Windows 10 Pro 64-bit
Web Browser:Chrome 55.0.2883.87
Applies to:Dream Maker
Status: Open

Issue hasn't been assigned a status value.
1. Hold shift.
This sets the selection startpoint at the cursor's current position.

2. Press the down arrow key a few times.
This moves the selection endpoint down a line for each button press.

3. Press the up arrow key once.
Before 511, this moves the selection endpoint up one line.
In 511, this moves the selection startpoint to the bottom, and then moves the endpoint up one line.

Was this change made on purpose? It doesn't make sense.

4. Release shift whenever you feel like it.
Bump, still happens.

Demo video:

When moving the selection up, it should not reset the starting point of the selection. As of 511, it does.
Yep, I've noticed this too. Rather a pain. I'll have to dig into it sometime to figure out what changed.