vickster
Squire
I never have feet flat when seated but I rarely slide forwards unless I know it's going to be a long wait eg level crossing when barriers have just come down. I seem to get snagged getting back onto the saddle if I do that 
I just tip the bike slightly to the left and tiptoe a bit more stably on ground, preferably planting my foot on pavement (obviously not always possible if turning right at lights etc). Long legs help this process
@Leemi1982 having the saddle too low will certainly exacerbate any knee issue

I just tip the bike slightly to the left and tiptoe a bit more stably on ground, preferably planting my foot on pavement (obviously not always possible if turning right at lights etc). Long legs help this process
@Leemi1982 having the saddle too low will certainly exacerbate any knee issue

) but, I wasn't sure how experienced the OP was. I am a believer in doing whatever suits our own individual physical limitations and will only ever off advice to somebody and certainly never utter that arrogant phrase "oy, you're doing that wrong" which some people regularly use freely 