How can I temporarily make the window I'm working on to be fullscreen in vim?

Ctrl+W_ will maximize a window vertically.

Ctrl+W| will maximize a window horizontally.

So far as I'm aware, there is no way to restore the previous layout after these actions, but Ctrl+W= will resize all windows to equal sizes.

An option could be to pursue the editing in a new tab. The following command opens the active buffer into a new tab allowing you to see the buffer in the hole vim window.

:tab split

And close the tab when you're done:

:tabc

Edit:

You can always use the following command to use tt as a shortcut (or better add it to your .vimrc):

:noremap tt :tab split<CR>

and close is when you're done :

:wq

If I understand what you're asking, I think you'll find the ZoomWin plugin helpful (GitHub). If you've got a bunch of split windows, and you want to temporarily make the current window the only visible one, you can hit <C-w>o. When you want to revert to the previous split state, hit <C-w>o again.

[Edit] Note on key mappings:

The default key mapping for this plugin is <C-w>o, but that conflicts with a default Vim key mapping. By default, that does :only, which makes the current window the only window. If you'd like to retain that functionality, you can remap ZoomWin to another key. I remap it to <C-w>w, because I like to use the :only option as well. Here's my mapping:

nnoremap <silent> <C-w>w :ZoomWin<CR>

Note that this also overrides a default Vim mapping, related to moving to other visible windows (:help CTRL-W_w), but I never used that one anyway.