[Solved] Maximising VirtualBox screen reduces VM resolution

[dabigjoe97]

The resolution of my VM is significantly reduced when running it in windowed mode, but only if the virtual box screen is maximised. By this I mean I can resize the virtual box window to any size and the VM scales properly (auto resizing is on and working), but when when I maximise the virtual box window (not to full screen mode but just so it is the size of the screen), the VM suddenly reduces in resolution. It works perfectly fine in full screen mode. I have attached images to show what's happening. Note in VM_Full.jpg that for some reason Virtual Screen 1 is resized to 800 x 600 rather than resizing to the actual window size.

Below is an extract from the log when I maximise the window - it seems to attempt the resize but then defaults to 800 x 600 instead... For reference I'm running Windows 10 Host with Ubuntu 16.04 LTS Guest on VirtualBox 5.2.6 r120293 with the latest guest additions installed.

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00:51:34.501401 GUI: UIMachineView::sltPerformGuestResize: Sending guest size-hint to screen 0 as 1920x975 if necessary
00:51:34.501461 VMMDev: SetVideoModeHint: Got a video mode hint (1920x975x32)@(0x0),(1;0) at 0
00:51:34.505968 VBVA: InfoScreen: [0] @0,0 1904x955, line 0x1dc0, BPP 0, flags 0x5
00:51:34.506000 Display::handleDisplayResize: uScreenId=0 pvVRAM=0000000011c1a000 w=1904 h=955 bpp=0 cbLine=0x1DC0 flags=0x5
00:51:34.506022 GUI: UIFrameBufferPrivate::NotifyChange: Screen=0, Origin=0x0, Size=1904x955, Sending to async-handler
00:51:34.506106 GUI: UIMachineView::sltHandleNotifyChange: Screen=0, Size=1904x955
00:51:34.506122 GUI: UIFrameBufferPrivate::handleNotifyChange: Size=1904x955
00:51:34.506146 GUI: UIFrameBufferPrivate::performResize: Size=1904x955, Using fallback buffer since no source bitmap is provided
00:51:34.507302 VBVA: InfoScreen: [0] @0,0 800x600, line 0xc80, BPP 0, flags 0x5
00:51:34.507336 Display::handleDisplayResize: uScreenId=0 pvVRAM=0000000011a45000 w=800 h=600 bpp=0 cbLine=0xC80 flags=0x5
00:51:34.507357 GUI: UIFrameBufferPrivate::NotifyChange: Screen=0, Origin=0x0, Size=800x600, Sending to async-handler
00:51:34.507425 VBVA: InfoScreen: [0] @0,0 800x600, line 0xc80, BPP 32, flags 0x1
00:51:34.507447 Display::handleDisplayResize: uScreenId=0 pvVRAM=0000000011a45000 w=800 h=600 bpp=32 cbLine=0xC80 flags=0x1
00:51:34.507500 GUI: UIFrameBufferPrivate::NotifyChange: Screen=0, Origin=0x0, Size=800x600, Sending to async-handler
00:51:34.508716 GUI: UIMachineView::storeGuestSizeHint: Storing guest-screen size-hint for screen 0 as 1904x955
00:51:34.510290 GUI: UIMachineView::sltHandleNotifyChange: Screen=0, Size=800x600
00:51:34.510315 GUI: UIFrameBufferPrivate::handleNotifyChange: Size=800x600
00:51:34.510319 GUI: UIFrameBufferPrivate::performResize: Size=800x600, Directly using source bitmap content
00:51:34.512300 GUI: UIMachineView::storeGuestSizeHint: Storing guest-screen size-hint for screen 0 as 800x600
00:51:34.531007 GUI: UIMachineView::sltHandleNotifyChange: Screen=0, Size=800x600
00:51:34.531049 GUI: UIFrameBufferPrivate::handleNotifyChange: Size=800x600
00:51:34.531406 GUI: UIMachineView::storeGuestSizeHint: Storing guest-screen size-hint for screen 0 as 800x600
00:51:34.543853 GUI: UIMediumEnumerator: Medium-enumeration finished!

[dabigjoe97]

So I found the solution to this problem, not entirely sure if it's a VB issue or comes from the host. Pressing Win+P would cycle the virtual machine between three different modes when in windowed mode: one where the screen was tiny as described in the original problem, one where it was well beyond the size of the screen, and another where it fit perfectly (hence fixing the problem!).

[socratis]

not entirely sure if it's a VB issue or comes from the host

The host.

[MrMikeB]

I don't know what caused it, but I was having a strange display issue. For a few days, I was using this command:

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VBoxManage.exe controlvm "Ubuntu 14.04" setvideomodehint 1918 1080 32

and it would fix the display issue, while using the actual display's width of 1920 would cause the HEIGHT (strangely) to be much more than it should have.
Using the windows+P key to cycle through the display modes, getting it back to the mode I use, seems to somehow have fixed the wrong-window-sizing when using Host+F to get into full-screen.