Only just installed VirtualBox with Ubuntu as a guest OS on my Win7 64-bit PC a couple of days ago, so a real beginner, and already having problems.

Started Ubuntu in VirtualBox today, and was told I had Ubuntu updates waiting, just like Windows Update. I said go ahead, and they were installed (53Mb). I had to reboot the Ubuntu guest OS afterwards. That's when my problems started. I discovered I needed to reboot the host OS, and re-install Guest Additions in VirtualBox, after which some of the problems went away, but I'm left with this annoying one:
Before the update, Ubuntu fitted nicely into whatever window size I gave it, scaling nicely and properly as you'd expect as I re-sized the window. After the update, it now insists on only being a rigid 4:3 aspect ratio, which doesn't work on my 16:9 1920x1080 screen. So within the maximised VirtualBox window I have white space at left and right of the Ubuntu display, and I have Ubuntu scroll bars for up and down. A quarter of the (Ubuntu) screen is off the bottom of the (VirtualBox) screen, and I have to use the scroll bars to see it.

Choosing Settings/Display shows that my display (unhelpfully described as 'VBX 0"') only has 4:3 resolution settings (max is 1600x1200). I've tried 'Sticky Edges' both On and Off (no idea what this means), but it doesn't make any difference.

In the VirtualBox Toolbar, under View I have 'Switch to Fullscreen', 'Switch to Scale Mode' and 'Adjust Window Size' available, but none of these help. There's also 'Switch to Seamless Mode' and 'Auto-resize Guest Display', but these are greyed out. I can't get a fully-resizeable Ubuntu display which automatically scales to fit the window, like I had before the update.

What am I doing wrong? Why has the Ubuntu Update seemingly wrecked the display? Can anybody help?