How do you secondary click on the Windows 10 taskbar when it is full?

If you have lots of running apps the Windows taskbar cannot be clicked with your secondary mouse button to access the context menu (unless you close apps to make space just to do this).

Is there another way?

Answer

Use utility buttons (yellow) or areas outside other buttons (red) .

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Easier:

If you did not hide standard areas indicated as yellow, they still carry general right-click menu items.

Harder:

If you disabled the above buttons, use the areas (highlighted as red) outside other buttons (Start, task buttons, notification icons, Action Center button, Show Desktop button). They are quite narrow but they work. A good indication of them is that no button or icon is being highlighted under your mouse pointer position.

Conditionally available areas

Next to notification icons

This special case applies only to screens with certain scaling which causes that the icons in the notification area do not fully cover the taskbar.
One pixel high area below notification icons (see the blue highlight). In case of vertical taskbar, it can be on any side, depending on the above factor, too. Check to find yours. If it works, it can be handy, because for example running the mouse to the bottom edge of the screen in the notification area can always find it.

On the inner edge of unlocked taskbar

If your taskbar is unlocked (Lock all taskbars in its context menu is unchecked), then the context menu is available everywhere on the inner (movable) edge of the taskbar. You can right-click everywhere where mouse pointer changes its shape into double arrow, indicating that you can move the edge and change the width of the taskbar.