Click on "Defines" button and add WITH_GTK2_IM, then check it. But then another problem arrives...
When rebuild IDE, this information popup:
Unable to find file "datetimepicker.pas".
If it belongs to your project, check search path in
Project -> Compiler Options -> Search Paths -> Other Unit Files. If this file belongs to a package, check the appropriate package compiler options. If this file belongs to Lazarus, make sure compiling clean. If the file belongs to FPC then check fpc.cfg. If unsure, check Project -> CompilerOptions -> Test
I do not use datetimepicker nor ATSynEdit - this is just after recompiling IDE with such parameter. And I have created similar thread few months ago, but there is no solution. And instead ATSynEdit which I used in the past without problem, now I need to use Lazarus' SynEdit which is full of bugs and memory leaks...
Lazarus is full of strange bugs...
For the author: is it this WITH_GTK2_IM parameter really important for functionality? In earlier versions that was not existed.