Set run-time cursor position in directory page dialog box - NSIS - nsis

Question

I want to add at the end of the path the installation folder name of my App. I do it successfully after clicking the Browse button but, after many tentatives, I couldn't do it if I modify the text directly in the textbox.
Function .onVerifyInstDir
Push $0
FindWindow $0 "#32770" "" $HWNDPARENT
GetDlgItem $0 $0 0x3FB
;in alternative for MUI >2.0 I could use directly $mui.DirectoryPage.Directory. Is it right?
;does path end with "\MyApp"?
StrLen $R1 "\${APP_FOLDER_NAME}"
StrCpy $R0 $INSTDIR "" -$R1
StrCmp $R0 "\${APP_FOLDER_NAME}" +2
;add "\MyApp" after browse button clicking (OK)
StrCpy $INSTDIR "$INSTDIR\${APP_FOLDER_NAME}"
;add "\MyApp" after typing directly into the textbox but the cursor position reset to the first character. Tried to solve saving the current cursor position and then reapply it (NOK)
SendMessage $0 ${EM_GETSEL} null $1
SendMessage $0 ${WM_SETTEXT} 0 "STR:$INSTDIR"
SendMessage $0 ${EM_SETSEL} $1 $1
FunctionEnd
I don't succeed of mantain the cursor where I'm modifying the path string and it reset always to the first char.
In a previous phase of install process I initialise the Install path as follow
StrCpy $INSTDIR "$APPDATA\${APP_FOLDER_NAME}"

Answer 1

NSIS is supposed to do this for you already.
From the docs:
... the part of this string following the last \ will be used if the user selects 'browse', and may be appended back on to the string at install time (to disable this, end the directory with a \ (which will require the entire parameter to be enclosed with quotes). If this doesn't make any sense, play around with the browse button a bit.
Meaning,
InstallDir "$ProgramFiles\MyApp"
is not the same as
InstallDir "$ProgramFiles\MyApp\"
Setting the text in .onVerifyInstDir is not officially supported but this code somewhat works:
!define APP_FOLDER_NAME MyApp
Page Directory
Page InstFiles
!include WinMessages.nsh
!include LogicLib.nsh
Function .onInit
StrCpy $INSTDIR "$APPDATA\${APP_FOLDER_NAME}"
FunctionEnd
Var InOnVerifyInstDir
Var SeenModal
Function .onVerifyInstDir
${IfThen} $InOnVerifyInstDir <> 0 ${|} Return ${|} ; Don't recurse into .onVerifyInstDir
!if ${MUI_SYSVERSION} >= 2.0
StrCpy $0 $mui.DirectoryPage.Directory
!else
FindWindow $0 "#32770" "" $hWndParent
GetDlgItem $0 $0 0x3FB
!endif
System::Call 'USER32::GetActiveWindow()p.r2'
System::Call 'USER32::GetFocus()p.r1'
${If} $1 P<> $0
${If} $hWndParent P<> $2
StrCpy $SeenModal 1
Return
${EndIf}
${If} $SeenModal = 0
Return
${EndIf}
${EndIf}
StrCpy $SeenModal ""
;does path end with "\MyApp"?
StrLen $R1 "\${APP_FOLDER_NAME}"
StrCpy $R0 $INSTDIR "" -$R1
${If} $R0 != "\${APP_FOLDER_NAME}"
StrCpy $InOnVerifyInstDir 1
StrCpy $INSTDIR "$INSTDIR\${APP_FOLDER_NAME}"
SendMessage $0 ${EM_GETSEL} "" "" $1
IntOp $1 $1 >> 16 ; shift hiword
IntOp $1 $1 & 0xffff ; mask possible sign bit
SendMessage $0 ${WM_SETTEXT} 0 "STR:$INSTDIR"
SendMessage $0 ${EM_SETSEL} $1 $1
StrCpy $InOnVerifyInstDir 0
${EndIf}
FunctionEnd
but I would still recommend just using InstallDir.