Bug 291141

It is probably a kwin bug. Restart of kwin fixed the problem (for a while). Maybe it is connected with https://bugs.kde.org/show_bug.cgi?id=289945

I believe I have this problem - my panel stopped appearing after updating to 4.8. It seems to be triggered by using present windows or desktop grid (it was triggered also by default alt-tab "brought over" from 4.7 - making kde use the new alt-tab stopped the triggering) only when switching desktops on edge is turned on. It affects both the autohidden panel and the one covered by windows. Restarting plasma-desktop, disabling switching desktops on edge or changing plasma theme fixes the problem until present windows or desktop grid is run again.

Still present in KDE 4.8.0. Present windows or Alt-tab cause it to stop appearing every time. Alt-F1 always brings it up though; kind of a crude work-around since you get the kickoff launcher too. Works better than restarting plasma-desktop; that has its own issues with system tray taking over the whole panel, getting progressively larger with each restart.

The Alt-F1 workaround works only when hidden panel contains kickoff launcher. Unfortunately, I've different panel hidden :-(

I can confirm that Presence windows effect cause this. After switch to Film strip, hidden panel works correctly. So maybe this is a kwin bug.

I just tested in 4.8.1. 
With the Film strip effect all works as should. The hidden panel pop up when I point mouse to the bottom of the screen. But after I enable the Present windows effect and use it (Alt + Tab), the hidden panel does not pop up anymore.

Is this a regression or did the same problem also exist with 4.7. There have been changes and we need to figure out whether this is a to be expected behavior or a bug.

I've not had the problem prior to 4.8 so I would consider it a regression. I was using Present Windows and Alt-Tab on 4.7 and prior with no problem.

I bet on bug #288791

(In reply to comment #7)
> Is this a regression or did the same problem also exist with 4.7. There have
> been changes and we need to figure out whether this is a to be expected
> behavior or a bug.

It work correctly in 4.7.

And strange is that switch off/on compositor does not help. Also restart kwin does not help. Only restart of plasma allow panel to pop up again.

(In reply to comment #11)
> And strange is that switch off/on compositor does not help. Also restart
> kwin does not help. Only restart of plasma allow panel to pop up again.

Because it's most likely a conflict about the electric border z-axis, tus a dupe.
if the panel never raises this proxy and kwin starts after plasma-desktop, you actually trigger the issue.

a) "kcmshell4 kwinscreenedges" ->what settings? (you can make a sceenshot in a readable language)
b) run konsole, run "xwininfo", move the mouse to a panel border, click and post the output. Ideally configure the panel to NOT take an entire screen width/height before

possible solutions/workarounds for now:
a) plasma uses 2px wide proxies
b) plasma triggers a raise on those proxies every n seconds
c) for certain conditions a patch to exclude not required electric borders exists, but that's only half cut.

proprietary solutions could be to have kwin a heuristic lookup about the windows touching borders (ie. make kwin check whether that might be a foreign electric border) or -better- plasma tags those windows with a prop. _KDE_WM property. Both won't work with other WMs.

Created attachment 69357 [details]
kwinscreenedges setting

==== Before ====
$ xwininfo

xwininfo: Window id: 0x100b028 (has no name)

  Absolute upper-left X:  659
  Absolute upper-left Y:  1170
  Relative upper-left X:  659
  Relative upper-left Y:  1170
  Width: 602
  Height: 30
  Depth: 0
  Visual: 0x21
  Visual Class: TrueColor
  Border width: 0
  Class: InputOnly
  Colormap: 0x0 (not installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: yes
  Corners:  +659+1170  -659+1170  -659-0  +659-0
  -geometry 602x30+659-0 

==== AFTER ====
$ xwininfo

xwininfo: Window id: 0x1800068 (has no name)

  Absolute upper-left X:  1
  Absolute upper-left Y:  1199
  Relative upper-left X:  1
  Relative upper-left Y:  1199
  Width: 1918
  Height: 1
  Depth: 0
  Visual: 0x21
  Visual Class: TrueColor
  Border width: 0
  Class: InputOnly
  Colormap: 0x0 (not installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: yes
  Corners:  +1+1199  -1+1199  -1-0  +1-0
  -geometry 1918x1+1-0

It's a dupe. The reason is the "always enabled" setting for desktop switching.
you likely use a purely horizontal desktop layout then?

*** This bug has been marked as a duplicate of bug 288791 ***

(In reply to comment #14)
> It's a dupe. The reason is the "always enabled" setting for desktop
> switching.
> you likely use a purely horizontal desktop layout then?
> 
> *** This bug has been marked as a duplicate of bug 288791 ***

No, I've standard 2x2 desktops layout.

Stupid question: How do you pop up the bottom panel then? Ultra careful mouse approaching?

(In reply to comment #16)
> Stupid question: How do you pop up the bottom panel then? Ultra careful
> mouse approaching?

Well sometime is not so easy but with some practice it is not a problem. You have to tick the border. Go down to the border and immediately move up a little. There is some delay before switch to another desktop. To go to desktop below just continue down. And yes, it means that panel is shown for a while during desktop switch.