After having added a countdown timer of 1 min named TIMER1, it is shown in the list within the timer plugin window. However, it does not run by itself and I don't see where to start it. Neither is there anything to see in the small white rectangle that is shown in the xfce4 panel.
Please try version 0.6.4. You probably ran into the bug in 0.6.3 that prevented the pop-up menu functioning correctly under some panels.
Okay can you tell me what version of XFCE you are using? When you left-click on the plugin area on the panel, you should see a popup menu with a list of timers and the "Start timer" entry. Works here in my daily-use system on a 4.6 panel; but I develop on an XFCE 4.8 system and it was working there, too.
Created attachment 4246 screenshot big arrow points to xfce-timer
Actually, it works only when left-clicking on the plugin area *outside the white rectangle. Can't you simply show a clock which is then clickable everywhere? I have added a screenshot to show you how unattractive the plugin looks like here.
But again, could you tell me your XFCE version? It sounds like the old behaviour (even pre-0.6.3). That white rectangle is the progressbar itself. Quoting from one of my earlier posts here: "In versions 0.6.2 and before, the widget structure was like plugin -> eventbox > hbox that holds the progressbar etc and the mouseclick event was connected to the eventbox. But this way the mouse clicks on the progressbar were not detected. In 0.6.3 I removed the eventbox and connected the click signals directly to the plugin widget itself. This code worked as expected in 4.8 panels but caused the behaviour you mention [mouse clicks not working] in 4.6. Since I couldn't find a code that works for both, in 0.6.4 used separate codes for 4.6 and 4.8 panels."
ah sorry that's xfce4-4.8 and xfce4-panel-4.8.6 on Gentoo Linux
Created attachment 4247 xfcetimer.c
I confirmed your problem on 4.8 (progressbar itself doesn't receive clicks in vertical panels of XFCE 4.8, but 4.6 works OK). I discovered another bug along the way and fixed it. Could you retry with the attachment above? I haven't tested it on 4.8 and it may not solve your problem, but let's give a try...