Bug 316040

Suspend the compositor before adding the screen (shift+alt+f12)
Does it then act as expected? (While i actually doubt so from the description)

In case please attach the output of
   qdbus org.kde.kwin /KWin supportInformation

Otherwise please post the output of "xrandr -q" before and after adding the screen.

Disable the compositor  improved the situation a litle bit, now I can see the wall paper on the main monitor and use the plasma desktop on it, once I enable composite, the main monitor black out again, and even with the compositor disabled, I cannot move window from the external monitor to the main monitor, the other way around somehow works fine. And restart kwin fixes everything. xrandr -q look normal:
Before adding the monitor:
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 8192 x 8192
VGA-0 disconnected primary (normal left inverted right x axis y axis)
LVDS-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080      60.0*+   50.0
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)
After adding a new monitor:
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 8192 x 8192
VGA-0 disconnected primary (normal left inverted right x axis y axis)
LVDS-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080      60.0*+   50.0
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 connected (normal left inverted right x axis y axis)
   1920x1080      60.0 +
   1280x1024      75.0     60.0
   1152x864       75.0
   1024x768       75.0     60.0
   800x600        75.0     60.3
   640x480        75.0     59.9
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)

The output of qdbus org.kde.kwin /KWin supportInformation can be find in the attachment

Created attachment 77701 [details]
qdbus org.kde.kwin /KWin supportInformation output

----- Snip ----------
Before adding the monitor:
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 8192 x 8192
After adding a new monitor:
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 8192 x 8192


DP-3 connected (normal left inverted right x axis y axis)
   1920x1080      60.0 +

----- /Snip ----------

DP-3 is connected, but not used.

Does the output change with restarting kwin?

That output is just after I connected the monitor, before enable it using xrandr, after issuing xrandr --output DP-3 --auto --left-of LVDS-0 --output LVDS-0 --auto --primary the output is:
Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 8192 x 8192
VGA-0 disconnected (normal left inverted right x axis y axis)
LVDS-0 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080      60.0*+   50.0  
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 509mm x 286mm
   1920x1080      60.0*+
   1280x1024      75.0     60.0  
   1152x864       75.0  
   1024x768       75.0     60.0  
   800x600        75.0     60.3  
   640x480        75.0     59.9  
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)

Also if I disable the external monitor after I fixed the dual screen display by restarting kwin, the resulting desktop is squeezed to like 2/3 of the main screen, the rest 1/3 is just black band.

The issue is likely the screen caching.
If you want, you can try moving local
static xcb_screen_t *s_screen = NULL;

to a global static
static xcb_screen_t *gs_screen = NULL;

and add some

inline void invalidateScreen() {
   gs_screen = NULL;
}

To call eg at events.cpp

if (e->type == Extensions::randrNotifyEvent() && Extensions::randrAvailable()) {
            XRRUpdateConfiguration(e);
            invalidateScreen();
...
}

or in workspace.cpp
void Workspace::desktopResized()
{
            invalidateScreen();
            ...
}

Hi Thomas, sorry for the late reply, I tried the changes you suggested but no improvement, the diff look like this:
diff --git a/kwin/geometry.cpp b/kwin/geometry.cpp
index 03085db..3f99c01 100644
--- a/kwin/geometry.cpp
+++ b/kwin/geometry.cpp
@@ -67,6 +67,7 @@ extern bool is_multihead;
  */
 void Workspace::desktopResized()
 {
+    invalidateScreen();
     QRect geom;
     for (int i = 0; i < QApplication::desktop()->screenCount(); i++) {
         //do NOT use - QApplication::desktop()->screenGeometry(i) there could be a virtual geometry
diff --git a/kwin/libkwineffects/kwinglobals.h b/kwin/libkwineffects/kwinglobals.h
index aa2681c..13276f5 100644
--- a/kwin/libkwineffects/kwinglobals.h
+++ b/kwin/libkwineffects/kwinglobals.h
@@ -158,10 +158,10 @@ KWIN_EXPORT xcb_timestamp_t xTime()
     return QX11Info::appTime();
 }
 
+static xcb_screen_t *s_screen = NULL;
 inline
 KWIN_EXPORT xcb_screen_t *defaultScreen()
 {
-    static xcb_screen_t *s_screen = NULL;
     if (s_screen) {
         return s_screen;
     }
@@ -190,6 +190,10 @@ KWIN_EXPORT int displayHeight()
     return screen ? screen->height_in_pixels : 0;
 }
 
+inline void invalidateScreen() {
+       s_screen = NULL;
+}
+
 /** @internal */
 // TODO: QT5: remove

I also tried to change event.cpp (but not tried both together), the result is the same. I only restarted kwin by run kwin --replace in konsole without a full logout/login.

try after XRRUpdateConfiguration(e); in events.cpp as well.
If it's not screen caching i frankly don't know what could cause any issues here ... except getting the geometry update, and calling displayWidth/Height before the structure behind the defaultScreen() pointer has updated.

Brute force:
add "XSync(display(), false);" on top of the defaultScreen() function.

Git commit c61081ffed38a2f124631d41cede6e2b839026c9 by Martin Gräßlin.
Committed on 11/03/2013 at 15:14.
Pushed by graesslin into branch 'master'.

Temporarily revert to XDisplayWidth/Height

We don't like broken master. Proper fix coming later on.

M  +8    -0    kwin/libkwineffects/kwinglobals.h

http://commits.kde.org/kde-workspace/c61081ffed38a2f124631d41cede6e2b839026c9