RelGotoPage function goes to next page instead of going back - nsis

Question

My current defined order of pages in the Installer are:
!insertmacro MUI_PAGE_WELCOME ; Welcome page
!insertmacro MUI_PAGE_DIRECTORY ; Select a directory
!define MUI_PAGE_CUSTOMFUNCTION_PRE CheckInstallDirectory ; Check selected directory
!insertmacro MUI_PAGE_COMPONENTS ; Choose install components
!define MUI_PAGE_CUSTOMFUNCTION_PRE SkipLicense ; Component1's check before license page
!insertmacro MUI_PAGE_LICENSE "license.txt" ; Component1's license
!define MUI_PAGE_CUSTOMFUNCTION_PRE SkipLicenseForComp2 ; Component2's check before license page
!insertmacro MUI_PAGE_LICENSE "License2.txt" ; Component2's license
!insertmacro CUSTOM_PAGES ; Custom install pages
!insertmacro MUI_PAGE_INSTFILES ; Install components' files
!insertmacro MUI_PAGE_FINISH ; Finish page
In the CheckInstallDirectory function, which is defined below, it checks if the end-user selected a directory that already houses an .exe that would otherwise be installed by the installer.
If the directory exists, the user is presented a pop-up message box with the "OK" and "CANCEL" buttons.
"OK" selection should simply skip the next few pages and go to MUI_PAGE_INSTFILES page.
"CANCEL" selection should return the user to the MUI_PAGE_DIRECTORY page.
Function CheckInstallDirectory
IfFileExists "$INSTDIR\Component1.exe" file_found end_of_check
file_found:
Var /GLOBAL ver_
${GetFileVersion} "$INSTDIR\Component1.exe" $ver_
StrCpy $0 $ver_ 1 ; Determine the major version of the installed exe (1st char of the returned string)
StrCpy $1 $ver_1 1 ; Determine the major version of the exe we are installing (1st char of the returned string)
${If} $0 == $1
; The major version between .exe's is the same
MessageBox MB_OKCANCEL|MB_ICONEXCLAMATION "A copy of this software is already installed in the selected directory.$\nSelect $\"OK$\" if you would like to update the software to the latest version.$\nOtherwise select $\"Cancel$\" to go back and select a different installation directory." IDOK OK IDCANCEL CANCEL
OK:
; User selected to update this software - continue with the installation like normal
StrCpy $R9 "7" ; Skip to the installation page
Call RelGotoPage
goto end_of_check
CANCEL:
StrCpy $R9 "-1" ; Go back to directory selection page
Call RelGotoPage
goto end_of_check
${Else}
; The major version between the .exe's is NOT the same - cannot update
MessageBox MB_OK "A copy of this software is already installed in the selected directory.$\nIt is not part of the latest major-version group and cannot be updated. To update this installation, uninstall and use the installer to install the latest version.$\nOtherwise, select$\"OK$\" to go back and select a different installation directory."
StrCpy $R9 "-1" ; Go back to directory selection page
Call RelGotoPage
goto end_of_check
${EndIf}
end_of_check:
FunctionEnd
I have found NSIS's description for how to move across installer pages here.
Function RelGotoPage
IntCmp $R9 0 0 Move Move
StrCmp $R9 "X" 0 Move
StrCpy $R9 "120"
Move:
SendMessage $HWNDPARENT "0x408" "$R9" ""
FunctionEnd
The problem that is occurring is that StrCpy $R9 "7" does successfully skip to the MUI_PAGE_INSTFILES page. Yet, StrCpy $R9 "-1" does not return to the previous page (which should be MUI_PAGE_DIRECTORY). Instead, if "CANCEL" is selected in that pop-up message, the installer goes to the next page, MUI_PAGE_COMPONENTS.
What is incorrect here?
Is there a better way to achieve the desired result?
Any help is sincerely appreciated! Thank you!

Answer 1

In the page pre-callback the page is not fully loaded and I assume that is why -1 does not work correctly. -1 does work if you use MUI_PAGE_CUSTOMFUNCTION_SHOW but the proper way to do this is to block the page change:
!include MUI2.nsh
!insertmacro MUI_PAGE_WELCOME ; Welcome page
!define MUI_PAGE_CUSTOMFUNCTION_LEAVE CheckInstallDirectory
!insertmacro MUI_PAGE_DIRECTORY
...
Function CheckInstallDirectory
;Snipped
MessageBox MB_OKCANCEL|MB_ICONEXCLAMATION "A copy of this software is already installed in the selected directory.$\nSelect $\"OK$\" if you would like to update the software to the latest version.$\nOtherwise select $\"Cancel$\" to go back and select a different installation directory." IDOK OK IDCANCEL CANCEL
OK:
; User selected to update this software - continue with the installation like normal
StrCpy $R9 "7" ; Skip to the installation page
Call RelGotoPage
goto end_of_check
CANCEL:
Abort
;Snipped
end_of_check:
FunctionEnd