I could confirm the issue (not properly setting TMemo.SelLength/SelStart and reflecting the changes in the control) with r18956 GTK2 on Suse.
GTK1 behaves as expected.
Unfortunately after upgrading to r19689 building Lazarus failed
Compiling graphics.pp
canvas.inc(88,24) Error: identifier idents no member "SetClipping"
graphics.pp(2489) Fatal: There were 1 errors compiling module, stopping
Fatal: Compilation aborted
make[1]: *** [alllclunits.ppu] Fout 1
make[1]: Leaving directory `/home/bart/svnroot/lazarus/lcl'
make: *** [lcl] Fout 2
bart@simenon:~/svnroot/lazarus>
I'll probably have to update my compiler?
When I succeed in upgrading to latest revision and the problem is still there, I'll file a bugreport in the bugtracker.
Bart