"Tile window to the right" > small gaps with screen edges

I use a dedicated shortcut or mouse drag to stick a window to the very right of my screen. When I drag the window to the right a red overlay shows me that the window will fill half-right of my screen. This overlay does not show any gap between window and the screen edge. When I use this shortcut or this mouse drag, the window is located on the right with small gap with the right edge and another small gap with the bottom edge (cf screenshot).

The odd thing is that it only occurs with urxvt windows. I can't reproduce this behaviour with Firefox, vscode, vlc...

λ  ~  xwininfo

xwininfo: Please select the window about which you
          would like information by clicking the
          mouse in that window.

xwininfo: Window id: 0x320000a "workspace-0"

  Absolute upper-left X:  961
  Absolute upper-left Y:  26
  Relative upper-left X:  1
  Relative upper-left Y:  26
  Width: 950
  Height: 1036
  Depth: 24
  Visual: 0x21
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x3200003 (installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +961+26  -9+26  -9-18  +961-18
  -geometry 94x54-8+0
λ  ~  xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   2560x1440     60.00 +  59.99    59.99    59.96    59.95
   1920x1440     60.00
   1856x1392     60.01
   1792x1344     60.01
   2048x1152     59.99    59.98    59.90    59.91
   1920x1200     59.88    59.95
   1920x1080     60.01*   59.97    59.96    59.93
   1600x1200     60.00
   1680x1050     59.95    59.88
   1600x1024     60.17
   1400x1050     59.98
   1600x900      59.99    59.94    59.95    59.82
   1280x1024     60.02
   1440x900      59.89
   1400x900      59.96    59.88
   1280x960      60.00
   1440x810      60.00    59.97
   1368x768      59.88    59.85
   1360x768      59.80    59.96
   1280x800      59.99    59.97    59.81    59.91
   1152x864      60.00
   1280x720      60.00    59.99    59.86    59.74
   1024x768      60.04    60.00
   960x720       60.00
   928x696       60.05
   896x672       60.01
   1024x576      59.95    59.96    59.90    59.82
   960x600       59.93    60.00
   960x540       59.96    59.99    59.63    59.82
   800x600       60.00    60.32    56.25
   840x525       60.01    59.88
   864x486       59.92    59.57
   800x512       60.17
   700x525       59.98
   800x450       59.95    59.82
   640x512       60.02
   720x450       59.89
   700x450       59.96    59.88
   640x480       60.00    59.94
   720x405       59.51    58.99
   684x384       59.88    59.85
   680x384       59.80    59.96
   640x400       59.88    59.98
   576x432       60.06
   640x360       59.86    59.83    59.84    59.32
   512x384       60.00
   512x288       60.00    59.92
   480x270       59.63    59.82
   400x300       60.32    56.34
   432x243       59.92    59.57
   320x240       60.05
   360x202       59.51    59.13
   320x180       59.84    59.32
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected primary (normal left inverted right x axis y axis)

Any clue ?

EDIT : this issue is reported for xfwm4 4.14.4, see my comment bellow for xfwm4 4.16.1