Sometimes (criteria to reproduce it yet unclear), trying to quick tile a window to 1/4 results in the window being resized to 1/2 instead.
The preview size is correct.

For details, see copy of discussion from IRC below and supportInformation after that.
Running current KF5 from git + Qt 5.4.0.

[17:02:31] <eliasp> so I have some quick-tiling issues and I feel like I know what's going on…
[17:03:35] <eliasp> when having a 2nd screen connected, it looks like quick-tile will use the total height (both screens) to calculate the height of windows dragged into a corner…
[17:03:35] <eliasp> so instead of covering 1/4 of a screen, their just quick-tiled 1/2 as if I'd dragged them to the left/right edge
[17:03:55] <eliasp> the preview looks correct, but once I drop the window, the height covers the whole screen
[17:04:12] <mgraesslin> hmm,let's see whether that also happens with horiz layout
[17:04:34] <mgraesslin> no horiz layout is correct
[17:04:47] <eliasp> let me nopaste my screen layout
[17:04:49] <mgraesslin> no it isn't
[17:04:52] <mgraesslin> quarter is broken
[17:04:56] <mgraesslin> but half is ok
[17:04:57] <eliasp> oh, ok… ;)
[17:05:20] <eliasp> this just started 2 weeks ago or so… can't cleary say yet when specifically
[17:05:25] <eliasp> didn't bisect yet or so
[17:05:34] --> kshitij__ (~kshitij@14.99.124.11) has joined #kwin
[17:05:37] <mgraesslin> 2 weeks ago was christmas
[17:05:45] <mgraesslin> I doubt there were any changes in that area
[17:05:47] <eliasp> bugs as x-mas gifts… nice ;)
[17:05:49] <eliasp> hehe, ok
[17:05:56] <mgraesslin> I suspect something
[17:06:07] <mgraesslin> can you report a bug? I'll look into it tomorrow
[17:06:11] <eliasp> sure, will do


kwin supportInformation:

Version
=======
KWin version: 5.1.95
Qt Version: 5.4.0

Operation Mode: X11 only

Decoration
==========
Plugin: org.kde.breeze
Theme: 
Blur: 0
onAllDesktopsAvailable: true
alphaChannelSupported: true
closeOnDoubleClickOnMenu: false
decorationButtonsLeft: 0, 2, 9
decorationButtonsRight: 6, 3, 4, 5
borderSize: 3
gridUnit: 10
font: Oxygen-Sans,10,-1,0,50,0,0,0,0,0
smallSpacing: 2
largeSpacing: 10

Options
=======
focusPolicy: 1
nextFocusPrefersMouse: true
clickRaise: true
autoRaise: false
autoRaiseInterval: 0
delayFocusInterval: 0
shadeHover: false
shadeHoverInterval: 250
separateScreenFocus: false
placement: 4
focusPolicyIsReasonable: true
borderSnapZone: 10
windowSnapZone: 10
centerSnapZone: 0
snapOnlyWhenOverlapping: false
showDesktopIsMinimizeAll: false
rollOverDesktops: true
focusStealingPreventionLevel: 1
legacyFullscreenSupport: false
operationTitlebarDblClick: 5000
operationMaxButtonLeftClick: 5000
operationMaxButtonMiddleClick: 5015
operationMaxButtonRightClick: 5014
commandActiveTitlebar1: 0
commandActiveTitlebar2: 30
commandActiveTitlebar3: 2
commandInactiveTitlebar1: 4
commandInactiveTitlebar2: 30
commandInactiveTitlebar3: 2
commandWindow1: 7
commandWindow2: 8
commandWindow3: 8
commandWindowWheel: 31
commandAll1: 10
commandAll2: 3
commandAll3: 14
keyCmdAllModKey: 16777250
showGeometryTip: true
condensedTitle: false
electricBorderMaximize: true
electricBorderTiling: true
electricBorderCornerRatio: 0.25
borderlessMaximizedWindows: false
killPingTimeout: 5000
hideUtilityWindowsForInactive: true
inactiveTabsSkipTaskbar: false
autogroupSimilarWindows: false
autogroupInForeground: true
compositingMode: 1
useCompositing: true
compositingInitialized: true
hiddenPreviews: 1
unredirectFullscreen: false
glSmoothScale: 2
colorCorrected: false
xrenderSmoothScale: false
maxFpsInterval: 16666666
refreshRate: 0
vBlankTime: 6000000
glStrictBinding: true
glStrictBindingFollowsDriver: true
glCoreProfile: false
glPreferBufferSwap: 101
glPlatformInterface: 1

Screen Edges
============
desktopSwitching: false
desktopSwitchingMovingClients: false
cursorPushBackDistance: 1x1
timeThreshold: 150
reActivateThreshold: 350
actionTopLeft: 0
actionTop: 0
actionTopRight: 0
actionRight: 0
actionBottomRight: 0
actionBottom: 0
actionBottomLeft: 0
actionLeft: 0

Screens
=======
Multi-Head: no
Active screen follows mouse:  no
Number of Screens: 2
Screen 0 Geometry: 528,1440,1920x1080
Screen 1 Geometry: 0,0,2560x1440

Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Ivybridge Mobile 
OpenGL version string: 3.0 Mesa 10.5.0-devel (git-ff1948a)
OpenGL platform interface: GLX
OpenGL shading language version string: 1.30
Driver: Intel
GPU class: IvyBridge
OpenGL version: 3.0
GLSL version: 1.30
Mesa version: 10.5
X server version: 1.16.2
Linux kernel version: 3.18.2
Direct rendering: Requires strict binding: yes
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace:  no

Loaded Effects:
---------------
zoom
slidingpopups
kwin4_effect_login
slide
screenshot
minimizeanimation
kwin4_effect_translucency
desktopgrid
kwin4_effect_maximize
kwin4_effect_fade
presentwindows
highlightwindow
kwin4_effect_dialogparent
blur
contrast
logout
dashboard
windowgeometry
startupfeedback
screenedge
kscreen

Currently Active Effects:
-------------------------
blur
contrast

Effect Settings:
----------------
zoom:
zoomFactor: 1.2
mousePointer: 0
mouseTracking: 0
enableFocusTracking: false
followFocus: true
focusDelay: 350
moveFactor: 20
targetZoom: 1

slidingpopups:
fadeInTime: 150
fadeOutTime: 250

kwin4_effect_login:

slide:

screenshot:

minimizeanimation:

kwin4_effect_translucency:

desktopgrid:
zoomDuration: 300
border: 10
desktopNameAlignment: 0
layoutMode: 0
customLayoutRows: 2
usePresentWindows: true

kwin4_effect_maximize:

kwin4_effect_fade:

presentwindows:
layoutMode: 0
showCaptions: true
showIcons: true
doNotCloseWindows: false
ignoreMinimized: false
accuracy: 20
fillGaps: true
fadeDuration: 150
showPanel: false
leftButtonWindow: 1
rightButtonWindow: 2
middleButtonWindow: 0
leftButtonDesktop: 2
middleButtonDesktop: 0
rightButtonDesktop: 0

highlightwindow:

kwin4_effect_dialogparent:

blur:
blurRadius: 12
cacheTexture: true

contrast:

logout:
useBlur: true

dashboard:
brightness: 0.5
saturation: 0.5
blur: false

windowgeometry:
handlesMoves: true
handlesResizes: true

startupfeedback:
type: 1

screenedge:

kscreen: