NSIS uninstall and install active at the same time - nsis

Question

I have an NSIS installer that calls an uninstaller to automatically uninstall the older version of the program.
http://nsis.sourceforge.net/Auto-uninstall_old_before_installing_new
Unfortunately, there is the possibility for the user to click on the install, and run the installer before running the uninstall, then proceed with the uninstall...
Is there any way to force the installer in the background while the uninstall is active ?
Edit: tried to disable the "Install" button,
ClearErrors
;disable the Install button
GetDlgItem $R1 $HWNDPARENT 0
EnableWindow $R1 0
...
done:
;disable the Install button
GetDlgItem $R1 $HWNDPARENT 0
EnableWindow $R1 0
I keep disabling the "Close" button on the uninstaller instead. I thought that this section is still in the installer...

Answer 1

I don't see how there can be any UI displayed by .onInit that the user can click on since the main window has not been created yet but if you moved the code somewhere else you could Hide/Show the installer with HideWindow+BringToFront (Basically ShowWindow $HWNDPARENT 0|1) or disable the buttons you don't want the user to click on with GetDlgItem+EnableWindow.

Answer 2

Based on the code snippet you linked to I think that you have no need to worry.
The .onInit function is executed prior to the installer UI being drawn on the screen, hence the uninstaller is going to be run before the user even has a chance to interact with the installer.
Just to make sure you are correctly understanding what that code is doing, when you run the new installer it is looking in the Registry to find the location of the Uninstaller executable of the old version, then popping a message box asking if you would like to uninstall the previous version. If you click ok then it will execute the uninstaller using the ExecWait instruction which will prevent the next instruction from being executed until the uninstaller returns (finishes). If the user clicks cancel it will then skip the uninstall of the old version.
Because all of this is done in the .onInit function, the User has no chance to interact with the new version before the old version is uninstalled.
The only way they could install the new version prior to uninstalling the old version is if they click on cancel when the messagebox comes up. If you would like to remove this option then you could change the MessageBox code too:
MessageBox MB_OK|MB_ICONEXCLAMATION \
"${PROGRAM_NAME} is already installed. $\n$\nClick `OK` to remove the \
previous version"
You also need to remove the IDOK uninst and Abort lines following the MessageBox instruction.
Hope this helps.
The Basic format of your script should be:
Function .onInit
code you linked to goes here
FunctionEnd
Section "Install"
install stuff goes here
SectionEnd
Section "Uninstall"
Uninstall stuff goes here
SectionEnd

Answer 3

call Uninstaller as following.
ExecWait '"$INSTDIR\Uninstall.exe" _?=$INSTDIR'
Then use BringToFront
It will hide Installer until Uninstaller Exists.