APN 2018-04-10 The following sample code for a custom sash handle was posted by debianuser on the Tclers chat. In particular it illustrates how to pass on the mouse events to the underlying paned window handler.
package require Tk
panedwindow .pw -orient horizontal
listbox .pw.l1
listbox .pw.l2
.pw add .pw.l1 .pw.l2
pack .pw -fill both -expand true
image create photo img::grip -data R0lGODlhBAAvAPEAALetnfXz7wAAAAAAACH5BAEAAAIALAAAAAAEAC8AAAIjRBwZwmKomjsqyVdXw/XSvn1RCFlk5pUaw42saL5qip6gnBUAOw==
label .pw.grip -image img::grip -cursor sb_h_double_arrow ;# this cursor works!!!
place .pw.grip -relx 1 -rely .5 -anchor w -in .pw.l1
bind .pw.grip <Button-1> { event generate .pw <Button-1> -x [expr %x+[winfo x .pw.grip]] -y [expr %y+[winfo y .pw.grip]] }
bind .pw.grip <B1-Motion> { event generate .pw <B1-Motion> -x [expr %x+[winfo x .pw.grip]] -y [expr %y+[winfo y .pw.grip]] }
wm protocol . WM_DELETE_WINDOW exit
vwait foreverThe above is for Tk's (not ttk) panedwindow widget but should work for ttk::panedwindows as well. Also see ttk::panedwindow for an alternative specific to it.