Fix T39911: Unpredictable behaviour when editing y-coordinate of right handle via Active Key panel (for aligned handles)

Editing the y-coordinate of the right handle of a keyframe via the Active Keyframe
panel in the Graph Editor, while both handles are selected and are both of type "aligned"
resulted in weird behaviours such as the x-coordinate of the right handle changing
(and rapidly starting to overshoot) but nothing else. However, this problem
doesn't occur when only a single handle is selected.

It turns out that the "order of computation" logic in calchandleNurb_intern() gets
confused in the case of both handles being selected, and results in a sub-optimal
handling of the right handle being the one that's been changed. We hack around this
here by temporarily making it so that just the right handle is selected when doing
the updates here.

1 files changed, 29 insertions, 2 deletions

diff --git a/source/blender/editors/space_graph/graph_buttons.c b/source/blender/editors/space_graph/graph_buttons.c
index d139ec0bfa2..8193008098e 100644
--- a/source/blender/editors/space_graph/graph_buttons.c
+++ b/source/blender/editors/space_graph/graph_buttons.c
@@ -255,6 +255,32 @@ static void graphedit_activekey_handles_cb(bContext *C, void *fcu_ptr, void *bez
 	graphedit_activekey_update_cb(C, fcu_ptr, bezt_ptr);
 }
 
+/* update callback for editing coordinates of right handle in active keyframe properties
+ * NOTE: we cannot just do graphedit_activekey_handles_cb() due to "order of computation"
+ *       weirdness (see calchandleNurb_intern() and T39911)
+ */
+static void graphedit_activekey_right_handle_coord_cb(bContext *C, void *fcu_ptr, void *bezt_ptr)
+{
+	BezTriple *bezt = (BezTriple *)bezt_ptr;
+	
+	/* original state of handle selection - to be restored after performing the recalculation */
+	const char f1 = bezt->f1;
+	const char f3 = bezt->f3;
+	
+	/* temporarily make it so that only the right handle is selected, so that updates go correctly 
+	 * (i.e. it now acts as if we've just transforming the vert when it is selected by itself)
+	 */
+	bezt->f1 = 0;
+	bezt->f3 = 1;
+	
+	/* perform normal updates NOW */
+	graphedit_activekey_handles_cb(C, fcu_ptr, bezt_ptr);
+	
+	/* restore selection state so that no-one notices this hack */
+	bezt->f1 = f1;
+	bezt->f3 = f3;
+}
+
 static void graph_panel_key_properties(const bContext *C, Panel *pa)
 {
 	bAnimListElem *ale;
@@ -351,15 +377,16 @@ static void graph_panel_key_properties(const bContext *C, Panel *pa)
 		
 		/* next handle - only if current is Bezier interpolation */
 		if (bezt->ipo == BEZT_IPO_BEZ) {
+			/* NOTE: special update callbacks are needed on the coords here due to T39911 */
 			uiItemL(col, IFACE_("Right Handle:"), ICON_NONE);
 			
 			but = uiDefButR(block, NUM, B_REDR, "X:", 0, 0, UI_UNIT_X, UI_UNIT_Y,
 			                &bezt_ptr, "handle_right", 0, 0, 0, -1, -1, NULL);
-			uiButSetFunc(but, graphedit_activekey_handles_cb, fcu, bezt);
+			uiButSetFunc(but, graphedit_activekey_right_handle_coord_cb, fcu, bezt);
 			
 			but = uiDefButR(block, NUM, B_REDR, "Y:", 0, 0, UI_UNIT_X, UI_UNIT_Y,
 			                &bezt_ptr, "handle_right", 1, 0, 0, -1, -1, NULL);
-			uiButSetFunc(but, graphedit_activekey_handles_cb, fcu, bezt);
+			uiButSetFunc(but, graphedit_activekey_right_handle_coord_cb, fcu, bezt);
 			uiButSetUnitType(but, unit);
 			
 			/* XXX: with label? */