Fix T43989: Sequencer - Ctrl snapping a sequencer strip does not work if you specify the x axis.

New 'strip' snapping was simply not computed in case of constrained transform, hence init
'0' value was used as frame offset in this case.

This commit reorganizes a bit that snapping, to keep it more 'confined' into `snapSequenceBounds()`
dedicated function. It still needs a minor hack (setting snapping mode to something else than
defualt `SCE_SNAP_MODE_INCREMENT`, to avoid this snapping to be called by contraint code).

Thanks to Antony for review and enhancements.

This fix should be backported to 2.74.

source/blender/editors/transform/transform.c

@@ -7311,40 +7311,28 @@ static void headerSeqSlide(TransInfo *t, const float val[2], char str[MAX_INFO_L
 	                    WM_bool_as_string((t->flag & T_ALT_TRANSFORM) != 0));
 }
 
-static void applySeqSlideValue(TransInfo *t, const float val[2], int frame)
+static void applySeqSlideValue(TransInfo *t, const float val[2])
 {
 	TransData *td = t->data;
 	int i;
-	TransSeq *ts = t->customData;
 
 	for (i = 0; i < t->total; i++, td++) {
-		float tvec[2];
-
 		if (td->flag & TD_NOACTION)
 			break;
 
 		if (td->flag & TD_SKIP)
 			continue;
 
-		copy_v2_v2(tvec, val);
-
-		mul_v2_fl(tvec, td->factor);
-
-		if (t->modifiers & MOD_SNAP_INVERT) {
-			td->loc[0] = frame + td->factor * (td->iloc[0] - ts->min);
-		}
-		else {
-			td->loc[0] = td->iloc[0] + tvec[0];
-		}
-
-		td->loc[1] = td->iloc[1] + tvec[1];
+		madd_v2_v2v2fl(td->loc, td->iloc, val, td->factor);
 	}
 }
 
 static void applySeqSlide(TransInfo *t, const int mval[2])
 {
 	char str[MAX_INFO_LEN];
-	int snap_frame = 0;
+
+	snapSequenceBounds(t, mval);
+
 	if (t->con.mode & CON_APPLY) {
 		float pvec[3] = {0.0f, 0.0f, 0.0f};
 		float tvec[3];
@@ -7352,7 +7340,6 @@ static void applySeqSlide(TransInfo *t, const int mval[2])
 		copy_v3_v3(t->values, tvec);
 	}
 	else {
-		snap_frame = snapSequenceBounds(t, mval);
 		// snapGridIncrement(t, t->values);
 		applyNumInput(&t->num, t->values);
 	}
@@ -7361,7 +7348,7 @@ static void applySeqSlide(TransInfo *t, const int mval[2])
 	t->values[1] = floor(t->values[1] + 0.5f);
 
 	headerSeqSlide(t, t->values, str);
-	applySeqSlideValue(t, t->values, snap_frame);
+	applySeqSlideValue(t, t->values);
 
 	recalcData(t);
 

source/blender/editors/transform/transform.h

@@ -626,7 +626,7 @@ typedef enum {
 void snapGridIncrement(TransInfo *t, float *val);
 void snapGridIncrementAction(TransInfo *t, float *val, GearsType action);
 
-int snapSequenceBounds(TransInfo *t, const int mval[2]);
+void snapSequenceBounds(TransInfo *t, const int mval[2]);
 
 bool activeSnap(TransInfo *t);
 bool validSnap(TransInfo *t);

source/blender/editors/transform/transform_snap.c

@@ -577,6 +577,10 @@ static void initSnappingMode(TransInfo *t)
 			t->tsnap.mode = SCE_SNAP_MODE_INCREMENT;
 		}
 	}
+	else if (t->spacetype == SPACE_SEQ) {
+		/* We do our own snapping currently, so nothing here */
+		t->tsnap.mode = SCE_SNAP_MODE_GRID;  /* Dummy, should we rather add a NOP mode? */
+	}
 	else {
 		/* Always grid outside of 3D view */
 		t->tsnap.mode = SCE_SNAP_MODE_INCREMENT;
@@ -2431,15 +2435,16 @@ void snapGridIncrement(TransInfo *t, float *val)
 	snapGridIncrementAction(t, val, action);
 }
 
-int snapSequenceBounds(TransInfo *t, const int mval[2])
+void snapSequenceBounds(TransInfo *t, const int mval[2])
 {
 	float xmouse, ymouse;
 	int frame;
 	int mframe;
+	TransData *td = t->data;
 	TransSeq *ts = t->customData;
 	/* reuse increment, strictly speaking could be another snap mode, but leave as is */
 	if (!(t->modifiers & MOD_SNAP_INVERT))
-		return 0;
+		return;
 
 	/* convert to frame range */
 	UI_view2d_region_to_view(&t->ar->v2d, mval[0], mval[1], &xmouse, &ymouse);
@@ -2450,7 +2455,7 @@ int snapSequenceBounds(TransInfo *t, const int mval[2])
 	if (!ts->snap_left)
 		frame = frame - (ts->max - ts->min);
 
-	return frame;
+	t->values[0] = frame - ts->min;
 }
 
 static void applyGridIncrement(TransInfo *t, float *val, int max_index, const float fac[3], GearsType action)