Okay, you definitely do have typewriter scrolling turned on in compose mode. There are two independent settings for this feature. Whenever you are looking at the main interface; the toolbar button and menu command, you are setting TS for standard editing. Changes made here will adjust how standard editing works and make no impact on Compose mode. To change compose, you need to be in compose—and then slide your mouse to the top of the screen so the menu comes up. Use Format/Options/Typewriter Scrolling to disable it; or you can use Ctrl-Cmd-T.
I am extremely sorry, but that doesn’t change a thing …
Scrivener -> Full Screen mode --> Menu as described --> Tick off TS
–> close document, quit Scriv (to be sure) --> reopen Scriv and document --> check menu, TS is still unchecked
–> nothing changes … still hopping around … 
OK I didn’t restart the computer, but that shouldn’t be required, should it?
(yes I seem to radiate some extremely strong anti-technology aura, yet that is something to go esotheric …)
Did you try the settings before you quit Scrivener? Perhaps the setting just isn’t saving for some reason. I’m not clear on why you restarted Scrivener in the middle of this—perhaps for some other reason—but ordinarily you do not need to restart the program after you change a setting. If you have having problems with settings sticking, try your test in a new blank project. Perhaps the settings file in your working project has become damaged. If things work as they should in the blank file, let me know.
I tried it right after unchecking it, then after closing-reopening the project and than after quitting and restarting Scriv.
Blank file: Sorry, same results. It is as if a main preferences file would be somehow sticky, yet I do not encounter “sticky prefs” with other settings, only with TS … at least I did recognize so.
What about console messages? Anything coming up as an error when you try to toggle the setting? You’ll need to use the Scrivener/Reveal Support Folder in Finder menu command, and check out the Logs folder in there. Check to see if the file has any content after toggling in compose mode and standard mode; On-Off for each so four events. Paste whatever you get.
Also, try turning on the “Standard full screen scroller” preference so you can see the scrollbar. Typewriter mode adds padding to the bottom of the document, so that it can keep the last line centred. This means that when the mode is toggled on and off, you should see a noticeable change in the size of the scroller—assuming you are testing in a document that isn’t mammoth in size (say just a bit taller than the screen).
OK …
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The log is completely empty. No content whatsoever in the file Consloe.log (even after changing modes 6-8 times).
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The size of the scoller does change. Yet the behaviour remains the same …
Maybe flushing the preferences (a shame) would be a solution? I may try that …
Is the problem in composition (full screen) mode, in the regular editor, or both? The setting is saved separately for each, so even if you have turned typewriter mode off in the main editor, when you enter compose mode, it may turn back on - you would need to turn it off explicitly for compose mode via the Format > Options menu too.
You could try flushing the project preferences, yes, though I don’t see why that would help since you’ve observed the same behaviour in a new blank project, that would indicate it isn’t a corruption problem in the original project’s UI file, which is why I suggested trying it. But anyway, to try that hold down the Option key on the File menu and select Close Project and Clear Interface Settings.
Another thing to try. Make sure TC is off in Composition (use the scrollbar trick if need be), and then visit the preference tab for Compose and click that button at the bottom, “Use current composition settings for new projects”. That’ll suck in all settings, including whether TC is on, for new projects. Then try making a new blank project again and without changing anything see if it still snaps around when you edit. If it does, check the Format/Options menu and verify there is no checkmark.
The application preferences file itself should be even further removed from the problem, since it only sets up the project default and has no impact on the day-to-day functioning of the project. However you can try that as well if you want. Maybe use the Manage menu first to save all your settings to a preset so you can get back to your settings once done with the experiment. I really doubt this will do anything though, it would make no sense at all for it to. The project UI file in the first paragraph would make much more sense.
Keith, I think initially that was the problem—TC was being left on in Compose. I’m pretty sure at this point we’ve got that point cleared though and they are using the menu to clear the setting while in compose mode.
let Scriv “suck in” current settings as you suggested, also tried deleting the prefs, still - no change, TS is on even when I opened a new blank project after deleting system preferences and ticking off TC in Compose mode. I must do something wrong on a basic level, yet I can’t imagine what …
Mysterious …