In webkitgtk browser 2.12 and 2.14 (haven't tried trunk or 2.16 yet), right clicking in a Google Sheets document does nothing. It should pop up a custom context menu that has options like deleting rows and cells. Instead it does nothing. Even the default context menu does not appear.
However, if you are on a Google Doc instead, its custom right-click menu appears as expected.
Also interesting, Google Sheets has a custom keyboard shortcut of Ctrl+Shift+\ to make the custom context menu appear. That works as expected.
So the issue seems to be related to handling the "contextmenu" event and only on Google Sheets. The context menu works properly if it is invoked using the keyboard shortcut.
Additional testing gives some more insights. Right clicking on a selected cell does nothing, as originally reported. However, a selected cell will have a small blue square in its bottom right corner. Right-clicking that blue square WILL bring up the context menu.
So the real issue and difference with other browsers is that in WebKit, you have to right click the small blue square, but on other browsers, you can right-click anywhere in the cell.