CatsAreGods

Look for the item MOVIE OPTIMIZED CONTROL in the movie settings menu, set to OFF. For a description see page 161 of the manual. Hope this helps.

Actually not a baby; the first generation hatched is smaller.

Fujifilm X-S10 with XF 70-300mm lens on extension tubes, 1/2000 handheld f/10 ISO 6400

Walking !

Shadows !!

Tuk Tuk Speed 🛺 

Is there a disadvantage to leaving it set to on?

My X-S10 is on V1.04 and functions perfectly.  Never had an issue updating firmware on any of my Fuji cameras using an SD card created on a PC. 😉

You probably need to switch off Preview Exposure in the Setup/screen setup menu.

Definitely try some pics in good light of a static subject on a tripod. If those don't come out tack-sharp, a visit to the shop is definitely in order, Good luck, I hope there's nothing wrong with your camera. I've had a different mirrorless system (Sony) for 6 years and there is still plenty of stuff I'm learning about this thing!

if all this settings , tips etc.. fail . then definitely contact fujifilm service ! because then it might be a manufacturing defect ,which will be replaced by the seller . your camera and lens is capable of capturing way too faster moving subjects than dogs .so wish you good luck !

I've been shooting the moon with both the 70-300 and a Tamron 500 mirror lens with 2x teleconverter and had no problems whatever. It's a compact zoom, not a long telephoto.

It does not look like a standard tripod collar with quick release will fit this lens. You may be able to use something like this:
https://www.bhphotovideo.com/c/product/554350-REG/Manfrotto_293_293_Telephoto_Lens_Support.html/?cnxclid=16173288597642086376410070301008005
But these are getting tricky to find. Once you know the size, you may be able to adapt what are called guide scope rings:
https://www.highpointscientific.com/telescope-accessories/mounts/telescope-mount-accessories/mounting-plates-tube-rings-accessories/telescope-mounting-rings
Once you get things worked out, show us some photos!

This has been a very interesting thread. I never heard of Dual ISO and almost thought it was an in-joke (like "blinker fluid" for cars). To return the favor, I found this awesome video that explains the details, but you will need at least 2 or 3 cups of coffee to keep up:
 

Seems like your question is more about using the camera as a webcam than about vlogging. There's a big discussion about this very subject in this very forum; one person even discusses that switcher. I found that the simplest solution was to bypass the buggy Fujifilm webcam software and just buy around $20 worth of cables:
 

There are dummy batteries for NP-W126, even a kit from Fuji.
I'm using one, let me know if you want the reference.

x-t2, samyang 100mm macro, godox tt350f

Nature is getting Spring ready to go.

And the second at 
300.0mm · ƒ/5.6 · 1/500s · ISO 160

First shot with XF70-300mmF4-5.6 R LM OIS WR at 100 mm f4.6 1/500s came out pretty well if I may say so 🙂

Are you using Focus Check?
The point of the zoomed view is so that you can verify (or fine tune the focus). 

Thanks @CatsAreGods I had somehow ignored or missed this very explicit reference in the manual that answers the question. However, the answer to "why doesn't the USB power connection eliminate any battery drain?"  remains a mystery. I think this model and the switch to USB-C was not as well engineered as we'd expect from a company like Fujifilm. 

Some handheld indoor shots at 300mm...first one at 1/5 second ISO 800, shot of my kit lens at ISO 1600 and (wait for it) 1/2 second exposure! X-S10 IBIS FTW!

It's ok. I have the X-T30 mounted on a microscope and just wanted to tether. Instead I've gone back to using the hdmi output.
Thanks
Louise