The bottom line with this issue is that many applications (including parts of Windows itself) do not support DPI scaling especially well, and the larger the scale factor, the more apparent the problems become. For comparison, scale factors of 125% or 150% are fairly common to use on notebooks with FHD panels (especially by users with less than perfect eyesight), but because of the massively higher DPI on the 15" QHD+ panel compared to ordinary displays, Windows 8.1 defaults to a scale factor of 200%. Unfortunately there isn't much to be done about applications that scale poorly other than to wait for updates to those applications, which may unfortunately require paid upgrades to newer versions than you may be using.
Note that poor support for scaling in general is a separate issue from scaling issues that occur when using the built-in QHD+ panel simultaneously with an external display. For more information on that, please see Multi-monitor Setup.
If you set Windows scaling to 200%, you will have the display real estate equivalent to 1600x900 resolution, simply with much sharper graphics, particularly text. Many users seem to set a custom scale factor of 166% (~1080p) for the added real estate, so play around with it to see where you like it best. Note that if you are using a multi-monitor setup, you'll want to read Multi-monitor Setup.
Please note that on an application-by-application basis, Windows 8.1 permits you to disable scaling; to toggle this setting, find the *.exe used to launch the program (hold the Windows key and tap the 'e' key to launch Windows Explorer). The *.exe is likely located in your C:/Program Files (x86)/ directory under the publisher's name. Once you've found the *.exe in question, right-click on it and go into Properties. There are 5 tabs across the top (General, Compatibility, Digital Signatures, Security, and Details); click on 'Compatibility' and look for the check box next to 'Disable display scaling on high DPI settings.' Click in that box, then try relaunching the game to see if your scaling issues have been improved.
Assassin's Creed IV Black FlagEditWhen running Assassin's Creed IV Black Flag at 1920x1080 resolution, the XPS 15 fails to scale the image up to fill the display. One user suggested dropping the resolution in Windows to 1920x1080, then launching the game, though in the only instance in which this was attempted it made no difference. Assassin's Creed IV is a DirectX 11 game (which seems to be part of the problem with scaling), and there is no option to switch to DX9 (which appears to be able to scale properly). After reinstalling the game, one user noticed that when launching Assassin's Creed IV - and before making any changes to the graphics (leaving the resolution to a default 640x480) - it scales properly. However, any subsequent changes to resolution break scaling. Kelsett (who originally posted this issue to the user forum at NotebookReview) provided the following solution for playing Assassin's Creed IV full screen at up to 1440x900 resolution:
Change the resolution manually in the following files:
C:\Users\%username%\Documents\Assassin's Creed IV Black Flag\Assassin4.ini
C:\Users\%username%\Documents\Assassin's Creed IV Black Flag\GFXSettings.AC4BFSP.xml
One user found that if he set his desktop resolution to 1600x900 before launching BF4, he could get 35+ fps on medium graphics and not have to suffer the failure of the game to scale up to fill the display.
Chrome struggles a bit with HiDPI displays; you may have to manually enable Chrome's HiDPI mode. Change your Chrome shortcut so that it launches with the command line parameter "--high-dpi-support=1" and you should have a much better experience. Another user commented that simply opening the right-side drop menu in Chrome and selecting "relaunch Chrome in desktop mode" was a persistent change that improved the browsing experience noticeably. That being said, other users have found that FireFox is more adept at scaling than Chrome as of January 2014.
Far Cry 3Edit
When running Far Cry 3 at 1920x1080 resolution, the XPS 15 fails to scale the image up to fill the display. One user suggested dropping the resolution in Windows to 1920x1080, then launching the game, but another user tried this and said it made no difference. One proven solution was to switch from DirectX 11 to DirectX 9 in the game settings.
Two Worlds IIEdit
When running Two Worlds II at maximum resolution, the XPS 15 fails to scale the image to fill the display. One user thought the problem might be solved by unchecking "Disable display scaling on high DPI settings" under the compatibility tab of the game .exe's file properties, but it turns out the only effective solution was to run the game with DirectX 9, as the DirectX 10 .exe resulted in scaling issues but the DirectX 9 version did not.