You will need to move the window in your viewport to access that button it either 1) you cannot increase the resolution of your screen or 2) reduce the window of the application.
This can be done rather easily when you hold the window action down then drag the window. Your mouse pointer then can be anywhere in the window.
Thus: hold the window action key, Super by default, and drag the window to see the button.
Alternatively, you may consider moving to a desktop environment where workspaces are visually connected.
Some background on how it works on Gnome Shell
In Gnome Shell, workspaces are different visual environments. So it is not that a window moved to the edge will "spill" to another workspace.
The behavior you expect is only there with multiple monitors. Additional monitors are spatial extensions of the first workspace.
By default in Gnome Shell, you can only switch workspaces on the primary monitor, on the second monitor whatever item from the first workspace you placed there (or that spills over from the left screen will remain visible. In recent versions, an option is exposed to have all monitors switch workspaces. So if you change to another workspace, the second monitor will be an extension of that workspace instead of remain on the first.
