"code" cell css not overflowing correctly

[braxtonmckee]

These two pages don't work correctly:

class ResizePanelWithCodeObject(CellsTestPage):
    def text(self):
        return "Should see a resizable panel with a horizontal split. The contents should be scrollable code panes."

    def cell(self):
        return cells.ResizablePanel(
            cells.Code("\n".join([f"top panel line {line}" for line in range(100)])),
            cells.Code(
                "\n".join([" " * 30 + f"bottom panel line {line}" for line in range(100)])
            ),
            ratio=0.50,
            split="horizontal",
        )


class ResizePanelWithCodeObjectInPanels(CellsTestPage):
    def text(self):
        return "Should see a resizable panel with a horizontal split. The contents should be scrollable code panes."

    def cell(self):
        return cells.ResizablePanel(
            cells.Panel(
                cells.Code("\n".join([f"top panel line {line}" for line in range(100)]))
            ),
            cells.Panel(
                cells.Code(
                    "\n".join([" " * 30 + f"bottom panel line {line}" for line in range(100)])
                )
            ),
            ratio=0.50,
            split="horizontal",
        )

[darth-cheney]

@braxtonmckee See PR #111