This bug has been around for a while. If you hold control before drag-selecting text to use Scintilla's rectangle selection feature, it only works if you start near an identifier. In other words, it won't work if you're starting in the middle of nowhere, or from characters such as
This is especially annoying because the key used to rectangle select with the mouse (control) differs from the key used to rectangle select with the keyboard (alt). It might help to support alt-drag for rectangle select as well (so control can be used in the case that alt is drowned out by some Linux window manager, but otherwise they're consistent).