ViewFrame: disable overlay scrolling for the GtkScrolledWindow
The overlay scrollbars get in the way when trying to place the text cursor with the mouse. The problem occurs with both the right-side scrollbar and the bottom scrollbar. It's especially annoying when text wrapping is disabled, to place the cursor at the last line. An ugly workaround was to place the cursor at a different character, then move the cursor with the keyboard. It's a GTK regression that was never properly fixed in gedit.