SUMMARY
When plasma panel has BOTTOM HORIZONTAL position most BOTTOM cursor position belongs to panel.
Mouse click on most bottom position works on taskbar as expected.
When plasma panel has TOP HORIZONTAL position most TOP cursor position belongs to panel.
When plasma panel has RIGHT VERTICAL position most RIGHT cursor position belongs to panel.

But when plasma panel has LEFT VERTICAL position most LEFT cursor position DOES NOT belong to panel.
And click on most left cursor position not working.

Investigation of this issue shows that kwin creates window of 1 pixel width on left position of screen.
For what this window?
No other window manager do this.

STEPS TO REPRODUCE
Try mouse click on last cursor position for every screen edge and corresponding taskbar position (left,top,right,bottom)

OBSERVED RESULT
For top,bottom,right position of plasma panel click on last corresponding cursor position works as expected.
For left position of plasma panel click on last corresponding cursor position is not working.

EXPECTED RESULT
For left position of panel most left cursor position should belong to panel


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: ArchLinux
KDE Plasma Version: 5.19.3
KDE Frameworks Version: 5.71
Qt Version: 5.15