IB2017’s issue has since been reported as RT #127120. The cause is that the global <<PasteSelection>> (using middle-click to paste currently selected text) event in older versions of Tcl/Tk (prior to 8.5.16 and 8.6.1) is incorrectly bound to the right mouse button. While the responsible code is in Tcl/Tk itself (not Tcl::pTk), it can effectively be patched at runtime, so a future Tcl::pTk version will likely do this automatically if an affected Tk version is detected.

Update: Tcl::pTk 0.95 includes the runtime patch for older Tcl/Tk.