compositor: avoid transmitting invalid lines on double scroll

This happens in an operation which both increases topline and also
inserts new lines somewhere in the remaining are. So before drawing any
line, win_update() is performing two grid_scroll operations.

===
A
B
C
D
E^
F
===

Consider that new line will be inserted after line E and screen also
scrolled up to line C. First the topline will be adjusted (x is the
scrolling region, ! invalid/empty space created by the scroll):

===
C   x
D   x
E^  x
F   x
!   x
!   x
===

and then space is inserted for the new line

===
C
D
E^
!   x
F   x
!   x
===

The problem is that we are now assuming that any invalid area ! created
by a scroll is filled with actual contents (by win_line etc) before it
is scrolled again. But in this case the last invalid line ! gets
scrolled. Ideally we should make win_update smarter and just scroll
valid lines for the later scroll (it is just wasteful to scroll the
larger area anyway), but for the 0.4 releasejust make
the compositor ignore such an invalid line (as it will get overdrawn
anyway later).

0 files changed, 0 insertions, 0 deletions