gitk: Keep the same commits visible as other commits come in

Since commits come in out of order and get sorted as we see them,
we can have commits coming in and being placed before the commits
that are visible in the graph display pane.  Previously we just
displayed a certain range of row numbers, meaning that when
incoming commits were placed before the displayed range, the
displayed commits were displaced downwards.  This makes it so
that we keep the same set of commits displayed, unless the user
explicitly scrolls the pane, in which case it scrolls as expected.

We do this by having a "target" commit which we try to keep in the
same visible position.  If commits have come in before it we scroll
the canvases by the number of rows that it has moved in the display
order.

This also fixes a bug in rowofcommit where it would test
cached_commitrow before possibly calling update_arcrows, which is
where cached_commitrow gets invalidated if things have changed.
Now we call update_arcrows if necessary first.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -734,9 +734,6 @@ proc rowofcommit {id} {
     global varcid varccommits varcrow curview cached_commitrow
     global varctok vtokmod
 
-    if {[info exists cached_commitrow($id)]} {
-	return $cached_commitrow($id)
-    }
     set v $curview
     if {![info exists varcid($v,$id)]} {
 	puts "oops rowofcommit no arc for [shortids $id]"
@@ -746,6 +743,9 @@ proc rowofcommit {id} {
     if {[string compare [lindex $varctok($v) $a] $vtokmod($v)] >= 0} {
 	update_arcrows $v
     }
+    if {[info exists cached_commitrow($id)]} {
+	return $cached_commitrow($id)
+    }
     set i [lsearch -exact $varccommits($v,$a) $id]
     if {$i < 0} {
 	puts "oops didn't find commit [shortids $id] in arc $a"
@@ -1299,7 +1299,7 @@ proc confirm_popup msg {
 }
 
 proc makewindow {} {
-    global canv canv2 canv3 linespc charspc ctext cflist
+    global canv canv2 canv3 linespc charspc ctext cflist cscroll
     global tabstop
     global findtype findtypemenu findloc findstring fstring geometry
     global entries sha1entry sha1string sha1but
@@ -1774,7 +1774,7 @@ proc canvscan {op w x y} {
 
 proc scrollcanv {cscroll f0 f1} {
     $cscroll set $f0 $f1
-    drawfrac $f0 $f1
+    drawvisible
     flushhighlights
 }
 
@@ -4538,21 +4538,50 @@ proc undolayout {row} {
     }
 }
 
-proc drawfrac {f0 f1} {
-    global canv linespc
+proc drawvisible {} {
+    global canv linespc curview vrowmod selectedline targetrow targetid
+    global need_redisplay cscroll
 
+    set fs [$canv yview]
     set ymax [lindex [$canv cget -scrollregion] 3]
     if {$ymax eq {} || $ymax == 0} return
+    set f0 [lindex $fs 0]
+    set f1 [lindex $fs 1]
     set y0 [expr {int($f0 * $ymax)}]
-    set row [expr {int(($y0 - 3) / $linespc) - 1}]
     set y1 [expr {int($f1 * $ymax)}]
-    set endrow [expr {int(($y1 - 3) / $linespc) + 1}]
-    drawcommits $row $endrow
+
+    if {[info exists targetid]} {
+	set r [rowofcommit $targetid]
+	if {$r != $targetrow} {
+	    # Fix up the scrollregion and change the scrolling position
+	    # now that our target row has moved.
+	    set diff [expr {($r - $targetrow) * $linespc}]
+	    set targetrow $r
+	    setcanvscroll
+	    set ymax [lindex [$canv cget -scrollregion] 3]
+	    incr y0 $diff
+	    incr y1 $diff
+	    set f0 [expr {$y0 / $ymax}]
+	    set f1 [expr {$y1 / $ymax}]
+	    allcanvs yview moveto $f0
+	    $cscroll set $f0 $f1
+	    set need_redisplay 1
+	}
     }
 
-proc drawvisible {} {
-    global canv
-    eval drawfrac [$canv yview]
+    set row [expr {int(($y0 - 3) / $linespc) - 1}]
+    set endrow [expr {int(($y1 - 3) / $linespc) + 1}]
+    if {$endrow >= $vrowmod($curview)} {
+	update_arcrows $curview
+    }
+    if {[info exists selectedline] &&
+	$row <= $selectedline && $selectedline <= $endrow} {
+	set targetrow $selectedline
+    } else {
+	set targetrow [expr {int(($row + $endrow) / 2)}]
+    }
+    set targetid [commitonrow $targetrow]
+    drawcommits $row $endrow
 }
 
 proc clear_display {} {