Alright - here's another rules question I've got after looking into making a Swiftstride Shifter. I was thinking of making her a Scout Rogue. Then I noticed the Shifted ability of the Swiftstrider has a very similar effect to the Scout Rogue's 3rd level ability - specifically the reaction when an enemy ends their turn within 5 ft.

How do you imagine they work? I'm going to start by saying I assume you simply have to choose one or the other - but given how similar they are - seems sucky if they didn't combine.
Quotes:

Shifting Feature

While shifted, your walking speed increases by 10 feet. Additionally, you can move up to 10 feet as a reaction when a creature ends its turn within 5 feet of you. This reactive movement doesn’t provoke opportunity attacks.

Skirmisher

Starting at 3rd level, you are difficult to pin down during a fight. You can move up to half your speed as a reaction when an enemy ends its turn within 5 feet of you. This movement doesn’t provoke opportunity attacks.

RAW, you choose to use one feature or the other.  Each provides its own separate, self-contained reaction; you pick which one to use, which is generally always going to be the Scout's save in very weird, ****y circumstances that were discussed to death in another thread on this similar subject.

As a DM, I'd be inclined to stick to RAW, or at least not allow the two speeds to simply additively stack.  Additive stacking means the shifter has 40 feet of movement, so half of that +10 feet is 30 feet of reactionary movement.  Ad Mobile and/or monk or barbarian levels, or various speed-boosting items, and the critter easily gains more movement as an automatic reaction than most things have as a full-turn move.  Normally that takes serious investment, or at least the expenditure of magical resources.  Giving it to somebody basically for free is asking for trouble I'm reasonably sure I wouldn't want in my games.

Well, they definitely don't stack. They are separate features that both consume your reaction to use, not features that queue off of having used a reaction. There are two distinctions that keep them from being just a worse/better version of the other.

Skirmisher is half of your speed. This is typically better as most circumstances will have you capable of moving 15+ feet, but there's a downside to it being dynamic. If you've been affected by something that causes your speed to drop, you could be moving less than 10 feet. It could be impossible to use the feature at all if your speed has been reduced to zero.

Shifting Feature is a static 10 feet. That's typically not as good as Skirmisher, but it has the upside of almost always being a possibility for usage. Even when reduced to zero speed, so long as you aren't affected by a condition that explicitly makes it impossible for you to move/take a reaction, it'll be usable in situations that Skirmisher isn't.

Hard to say which one is functionally the best, but I think Shifting Feature has the most theoretical applicability.

Yea that's basically as I expected. Still kind of sucky to double up on the same feature. Kind of like a Swashbuckler Rogue with Mobile Feat or Fiend Warlock with Long Death Monk.

I'm not sure about being able to use Swiftstrider's 10 ft movement if your speed is 0. It does say "move" and "up to". The other thing of course is that the Shifting Feature is temporary - once per short rest - whereas Scout is permanent.

Quote from Emmber >>

Yea that's basically as I expected. Still kind of sucky to double up on the same feature. Kind of like a Swashbuckler Rogue with Mobile Feat or Fiend Warlock with Long Death Monk.

I'm not sure about being able to use Swiftstrider's 10 ft movement if your speed is 0. It does say "move" and "up to". The other thing of course is that the Shifting Feature is temporary - once per short rest - whereas Scout is permanent.

 Aaah... I didn't think about shifting not always being in effect. That's a good point which muddles things back to "I can't really say either is better".

The thing about movement & reactions is they don't care about what your normal speed is, or how much of it you may have already used during your turn. They're either based on your current maximum speed, or a flat value of movement regardless of speed. Shifting allows you to move a flat value as a reaction. Things that prevent you from taking that reaction at all (or having literally nowhere to go) shut it down, but things that only modify your current max speed (like Sentinel) do not.

That's a really good point from Sigred, I can't really see a way to explicitly tie that reaction move to one or more of the character's Speeds.... There may be features that say you cannot move at all, and being Prone is still going to make every foot of movement cost 2 feet of that 10-foot pool... but certain effects like getting whacked by Sentinel won't stop you from taking that 10-foot reaction move from Shifter, but would absolutely stop your Scout reaction. 

Yea - I can see the reasoning and it makes sense - but I’d feel really dirty and cheap trying to convince a DM of that mid-game if they’d just hit me with Sentinel.