How to prevent scrolling while entering text in a text entry box in a Fluid app?
I have had this too. It is caused due the live spell checking service responding late while you are typing.

Apparently, while replacing a mistyped word, the service takes focus of the text box. Usually, this happens so quickly that you don't notice it. But sometimes, the service takes too much time (relatively) to process a word while you are still typing. And if you press Spacebar* when this happens, the page moves up/down.

When this first happened, I thought it  was totally dependent on the performance of your computer and there was no specific way to solve it. 

Safe Reboot - try this first:

One day, I accidentally activated Safe Boot. For me, this stopped the problem from happening, even after I rebooted normally. Aside from its normal uses, Safe Boot also clears out caches, repairs permissions, and performs other maintenance tasks. That is why you see a progress bar on a Safe Boot.  But that method doesn't work for everyone.  

Turn off AutoCorrect - here's a solution that should work if that doesn't solve it:

This solution requires you to turn off auto-correct for the app. You can still keep spell-check on, so it will still highlight misspellings; it just won't automatically change them.


Go to the affected app.
Click in the box where you're having trouble when you enter text, to put the cursor there.
In the menu bar, select Edit > Spelling and Grammar > Uncheck Correct spelling automatically.  You can leave Check spelling when typing checked, so it will continue to show you misspellings as you type.


* Pressing the Spacebar when a text box isn't in focus sends a scroll down action to the current page. And pressing Shift + Spacebar sends a scroll up.