Cloning an editor tab to a new window, then changing tab's order of original window leads to a crash

Original Reporter info from Mantis: JuhaManninen @JuhaManninen

Reporter name: Juha Manninen

Description:

It can be reproduced with many different files, one combination is explained in steps to reproduce.
I remember seeing this bug for a long time already.

I tested with LCL-QT and Win32. Same crash in both.
GTK2 now has another bug, the popup menu of editor tabs is gone. Could not test there.

Steps to reproduce:

Open project lazarus.lpi.
Open units Main and Project from ide/ directory into an editor window.
Right click the Project editor tab and do "Clone to new window". New editor window was created.
Go back to the original editor window and move Project tab to left. Now it should be on the left side of Main tab.
Right click the Project tab again -> crash!

Additional information:

Program received signal SIGFPE, Arithmetic exception.
0x0000000000496c99 in RAISEGDBEXCEPTION (MSG=0x1766e10 'TIDEMenuSection.UpdateNeedTopSeparator inconsistency') at lclproc.pas:902
902	  if (length(Msg) div (length(Msg) div 10000))=0 then ;
(gdb) bt
#0  0x0000000000496c99 in RAISEGDBEXCEPTION (MSG=0x1766e10 'TIDEMenuSection.UpdateNeedTopSeparator inconsistency') at lclproc.pas:902
#1  0x00000000009b813b in UPDATENEEDTOPSEPARATOR (parentfp=0x7fffffffc370) at menuintf.pas:996
#2  0x00000000009b71d5 in UPDATEMENUSTRUCTURE (this=0x7fffddbe7780) at menuintf.pas:1107
#3  0x00000000009b88bf in ITEMVISIBLEACTIVECHANGED (this=0x7fffddbe7780, ANITEM=0x7fffca8782a0) at menuintf.pas:1271
#4  0x00000000009b8ee5 in INSERT (this=0x7fffddbe7780, INDEX=0, ANITEM=0x7fffca8782a0) at menuintf.pas:1344
#5  0x00000000009b8d9c in ADDLAST (this=0x7fffddbe7780, ANITEM=0x7fffca8782a0) at menuintf.pas:1333
#6  0x00000000009b4d12 in REGISTERIDEMENUCOMMAND (PARENT=0x7fffddbe7780, NAME=0x7fffc9bf67d8 'MoveToWindow1', CAPTION=0x7fffd0943d78 'Source Editor (2)', ONCLICKMETHOD=..., 
    ONCLICKPROC={procedure (TOBJECT)} 0x7fffffffc530, COMMAND=0x0, RESOURCENAME=0x0, USERTAG=0) at menuintf.pas:522
#7  0x0000000000b530c2 in TOWINDOW (parentfp=0x7fffffffc870, ASECTION=0x7fffddbe7780, OPNAME=0x178af30 'MoveToWindow', ONCLICKMETHOD=..., WINFORFIND=false)
    at sourceeditor.pp:6500
#8  0x0000000000b51fad in TABPOPUPMENUPOPUP (this=0x7fffd12a7bd0, SENDER=0x7fffd1737640) at sourceeditor.pp:6546
#9  0x00000000006f0fb8 in DOPOPUP (this=0x7fffd1737640, SENDER=0x7fffd1737640) at include/popupmenu.inc:37
#10 0x00000000006f1300 in POPUP (this=0x7fffd1737640, X=584, Y=131) at include/popupmenu.inc:74
#11 0x00000000006f1250 in POPUP (this=0x7fffd1737640) at include/popupmenu.inc:66
#12 0x0000000000b5bda9 in NOTEBOOKMOUSEUP (this=0x7fffd12a7bd0, SENDER=0x7fffd12a9ed0, BUTTON=MBRIGHT, SHIFT=..., X=33, Y=-15) at sourceeditor.pp:8390
#13 0x000000000069a52f in MOUSEUP (this=0x7fffd12a9ed0, BUTTON=MBRIGHT, SHIFT=..., X=33, Y=-15) at include/control.inc:4222
#14 0x0000000001146d6a in MOUSEUP (this=0x7fffd12a9ed0, BUTTON=MBRIGHT, SHIFT=..., X=33, Y=-15) at extendednotebook.pas:182
#15 0x0000000000693716 in DOMOUSEUP (this=0x7fffd12a9ed0, MESSAGE=..., BUTTON=MBRIGHT) at include/control.inc:2181
#16 0x0000000000694551 in WMRBUTTONUP (this=0x7fffd12a9ed0, MESSAGE=...) at include/control.inc:2646

Mantis conversion info:

Mantis ID: 29394
OS: Xubuntu
OS Build: 15.10
Platform: Linux
Version: 1.7 (SVN)
Fixed in revision: 51284 (#c7cf6925)
Target version: 1.6