Today I tested the daily build installing in another directory. When I started it, I correctly configured the paths.
The LazActiveX module was checked and the Lazarus IDE compiled without problems, but any applications that I started with it trowed a SIGILL on start. I don't know the exact cause.
For my flickering problem, I searched a bit in the constants, this is the solution :
SetWindowLong(Handle, GWL_STYLE, GetWindowLong(Handle, GWL_STYLE) or WS_CLIPCHILDREN or WS_CLIPSIBLINGS);
SetWindowLong(Handle, GWL_EXSTYLE, GetWindowLong(Handle, GWL_EXSTYLE) or WS_EX_COMPOSITED);
Adding this in the FormCreate event will make the window resize smoothly.
Maybe this could be interesting to solve other flickering problems.