Handle coincident page marks better

The `ReplaceSelectionsWith` function handles keeping page marks
in approximately the same location when a Search/Replace
operation spans a page boundary (by maintaining the relative
proportion of text each side of the page boundary).

This did not cope correctly when there were two or more
page marks at the same location (i.e. blank pages). The page
marks could get reversed, so that 015 came before 014, for
example (although still both at the same location).

Avoiding changing the mark gravity (whether the mark goes
left or right of text inserted at the mark) fixes this for marks within
the S/R region. Temporarily setting "right" gravity for page marks
that immediately follow the S/R region keeps those page marks
correct.

src/lib/Guiguts/TextUnicode.pm

@@ -278,17 +278,19 @@ sub ReplaceSelectionsWith {
             push( @lengths, $fmlen );
         }
 
-        # Set gravity of page marker after string to "right" to keep it after string
-        # It doesn't matter if it's not immediately after - no harmful effect since restored later
-        my $end_mark = "";    # Assume there isn't one
+        # Find page markers immediately after string so we can set their gravity
+        # to "right" to keep them after string
+        my @endmarks = ();
         $mark = $last;
         while ( $mark = $w->markNext($mark) ) {
             if ( $mark =~ m{Pg(\S+)} ) {    # Only look at page markers
-                $end_mark = $mark;
-                $w->markGravity( $end_mark, 'right' );
-                last;
+                last if $w->compare( $mark, ">", $last );
+                push( @endmarks, $mark );
             }
         }
+        for $mark (@endmarks) {
+            $w->markGravity( $mark, 'right' );
+        }
 
         if ( !@lengths ) {    # No page markers, so simply insert new and delete old text
             $w->insert( $last, $new_text );
@@ -312,28 +314,28 @@ sub ReplaceSelectionsWith {
                 last if $w->compare( $mark, '<', $first );    # Stop if before start of old string
                 next if ( $mark eq $prev );                   # Skip if we find same position again
 
-                $w->markGravity( $prev, 'right' ) if ( $prev =~ m{Pg(\S+)} );    # Keep page marker to right of replaced string
-                $w->delete( $mark, $prev );
-                $w->insert(
-                    $mark,
-                    substr(
-                        $new_text, $lengths[ $idx + 1 ], $lengths[$idx] - $lengths[ $idx + 1 ]
-                    )
-                );
-                $w->markGravity( $prev, 'left' ) if ( $prev =~ m{Pg(\S+)} );     # Restore page marker behaviour
+                my $insstr =
+                  substr( $new_text, $lengths[ $idx + 1 ], $lengths[$idx] - $lengths[ $idx + 1 ] );
+                my $inslen = length($insstr);
+                $w->markSet( 'mark_del_end', $prev );
+                $w->insert( $mark, $insstr );
+                $w->delete( "$mark + $inslen c", 'mark_del_end' );
                 $prev = $mark;
                 ++$idx;
             }
 
             # delete final (first) chunk and insert remainder
-            $w->markGravity( $prev, 'right' ) if ( $prev =~ m{Pg(\S+)} );    # Keep page marker to right of replaced string
-            $w->delete( $first, $prev );
-            $w->insert( $first, substr( $new_text, 0, $lengths[$idx] ) );
-            $w->markGravity( $prev, 'left' ) if ( $prev =~ m{Pg(\S+)} );     # Restore page marker behaviour
+            my $insstr = substr( $new_text, 0, $lengths[$idx] );
+            my $inslen = length($insstr);
+            $w->markSet( 'mark_del_end', $prev );
+            $w->insert( $first, $insstr );
+            $w->delete( "$first + $inslen c", 'mark_del_end' );
         }
 
-        # restore gravity for page mark after replaced text
-        $w->markGravity( $end_mark, 'left' ) if $end_mark;
+        # restore gravity for page marks after replaced text
+        for $mark (@endmarks) {
+            $w->markGravity( $mark, 'left' );
+        }
     }
 
     # set the insert cursor to the end of the last insertion mark
@@ -343,6 +345,7 @@ sub ReplaceSelectionsWith {
     for ( my $i = 0 ; $i < $range_total ; $i++ ) {
         $w->markUnset( 'mark_sel_' . $i );
     }
+    $w->markUnset('mark_del_end');
 
     $w->addGlobEnd;
 }