SelTrans: fix inconsistencies with _norm, _grip, and rotation center

There are some inconsistencies when displaying the _norm, _grip, and
rotation center handles:

  1. _norm and rotation center are not displayed on a black background;

  2. _grip is never displayed, even though the code updates its position
     and even changes its shape depending on the operation performed.

Fix these issues by using the XOR blending mode and, since the handles
are only visible when dragging, set their color to 0xff0000b0 which is
the color used by other handles when they are in the
SP_KNOT_STATE_DRAGGING state.

Moreover to have _grip properly blended when HANDLE_CENTER is moved
_norm must be hidden, otherwise the two control point would overlap
(both in shape and position) and the XOR blending would produce an
unintuitive result.

Finally, to effectively hide _norm, move forcing handles visibility
_after_ grab() because grab() shows both _norm and _grip by default.

Fixed bugs:
  - https://bugs.launchpad.net/inkscape/+bug/1707250

1 files changed, 7 insertions, 5 deletions

diff --git a/src/seltrans.cpp b/src/seltrans.cpp
index 1afb71f68..9e18bbe90 100644
--- a/src/seltrans.cpp
+++ b/src/seltrans.cpp
@@ -131,13 +131,13 @@ Inkscape::SelTrans::SelTrans(SPDesktop *desktop) :
     _norm = sp_canvas_item_new(desktop->getControls(),
                                SP_TYPE_CTRL,
                                "anchor", SP_ANCHOR_CENTER,
-                               "mode", SP_CTRL_MODE_COLOR,
+                               "mode", SP_CTRL_MODE_XOR,
                                "shape", SP_CTRL_SHAPE_BITMAP,
                                "size", 13.0,
                                "filled", TRUE,
                                "fill_color", 0x00000000,
                                "stroked", TRUE,
-                               "stroke_color", 0x000000a0,
+                               "stroke_color", 0xff0000b0,
                                "pixbuf", handles[12],
                                NULL);
 
@@ -150,7 +150,7 @@ Inkscape::SelTrans::SelTrans(SPDesktop *desktop) :
                                "filled", TRUE,
                                "fill_color", 0xffffff7f,
                                "stroked", TRUE,
-                               "stroke_color", 0xffffffff,
+                               "stroke_color", 0xff0000b0,
                                "pixbuf", handles[12],
                                NULL);
 
@@ -727,12 +727,16 @@ void Inkscape::SelTrans::handleClick(SPKnot */*knot*/, guint state, SPSelTransHa
 
 void Inkscape::SelTrans::handleGrab(SPKnot *knot, guint /*state*/, SPSelTransHandle const &handle)
 {
+    grab(knot->position(), handle.x, handle.y, FALSE, FALSE);
+
+    // Forcing handles visibility must be done after grab() to be effective
     switch (handle.type) {
         case HANDLE_CENTER:
             g_object_set(G_OBJECT(_grip),
                          "shape", SP_CTRL_SHAPE_BITMAP,
                          "size", 13.0,
                          NULL);
+            sp_canvas_item_hide(_norm);
             sp_canvas_item_show(_grip);
             break;
         default:
@@ -744,8 +748,6 @@ void Inkscape::SelTrans::handleGrab(SPKnot *knot, guint /*state*/, SPSelTransHan
             sp_canvas_item_show(_grip);
             break;
     }
-
-    grab(knot->position(), handle.x, handle.y, FALSE, FALSE);
 }