Fix inaccuracy of SCREEN_OT_area_split when snap to midpoint and adjacent.

By default when moving a edge of the screen it always snaps to an invisible grid with unit of 4 pixels.
This was also affecting the snap to the midpoint and adjacent.
The solution was to make the snap to areagrid optional and use values of `origmin` and `origsize` that match the transformations in screen_edit.c.

2 files changed, 64 insertions, 44 deletions

diff --git a/source/blender/editors/screen/screen_edit.c b/source/blender/editors/screen/screen_edit.c
index da7bc9359dc..06f45fa0d00 100644
--- a/source/blender/editors/screen/screen_edit.c
+++ b/source/blender/editors/screen/screen_edit.c
@@ -226,25 +226,25 @@ static short testsplitpoint(ScrArea *sa, char dir, float fac)
 	CLAMP(fac, 0.0f, 1.0f);
 	
 	if (dir == 'h') {
-		y = sa->v1->vec.y + fac * (sa->v2->vec.y - sa->v1->vec.y);
-		
+		y = sa->v1->vec.y +
+		        round_fl_to_short(fac * (float)(sa->v2->vec.y - sa->v1->vec.y));
+
 		if (y - sa->v1->vec.y < area_min_y)
 			y = sa->v1->vec.y + area_min_y;
 		else if (sa->v2->vec.y - y < area_min_y)
 			y = sa->v2->vec.y - area_min_y;
-		else y -= (y % AREAGRID);
-		
+
 		return y;
 	}
 	else {
-		x = sa->v1->vec.x + fac * (sa->v4->vec.x - sa->v1->vec.x);
-		
+		x = sa->v1->vec.x +
+		        round_fl_to_short(fac * (float)(sa->v4->vec.x - sa->v1->vec.x));
+
 		if (x - sa->v1->vec.x < area_min_x)
 			x = sa->v1->vec.x + area_min_x;
 		else if (sa->v4->vec.x - x < area_min_x)
 			x = sa->v4->vec.x - area_min_x;
-		else x -= (x % AREAGRID);
-		
+
 		return x;
 	}
 }
@@ -609,7 +609,7 @@ static void screen_vertices_scale(
 
 	if (screen_size_x_prev != screen_size_x || screen_size_y_prev != screen_size_y) {
 		const float facx = ((float)screen_size_x - 1) / ((float)screen_size_x_prev - 1);
-		const float facy = ((float)screen_size_y) / ((float)screen_size_y_prev);
+		const float facy = ((float)screen_size_y - 1) / ((float)screen_size_y_prev - 1);
 
 		/* make sure it fits! */
 		for (sv = sc->vertbase.first; sv; sv = sv->next) {
@@ -617,7 +617,7 @@ static void screen_vertices_scale(
 			CLAMP(sv->vec.x, screen_rect->xmin, screen_rect->xmax - 1);
 
 			sv->vec.y = screen_rect->ymin + round_fl_to_short((sv->vec.y - min[1]) * facy);
-			CLAMP(sv->vec.y, screen_rect->ymin, screen_rect->ymax);
+			CLAMP(sv->vec.y, screen_rect->ymin, screen_rect->ymax - 1);
 		}
 	}
 
diff --git a/source/blender/editors/screen/screen_ops.c b/source/blender/editors/screen/screen_ops.c
index b5729a44034..dd808d122d2 100644
--- a/source/blender/editors/screen/screen_ops.c
+++ b/source/blender/editors/screen/screen_ops.c
@@ -1185,6 +1185,8 @@ typedef struct sAreaMoveData {
 	enum AreaMoveSnapType {
 		/* Snapping disabled */
 		SNAP_NONE = 0,
+		/* Snap to an invisible grid with a unit defined in AREAGRID */
+		SNAP_AREAGRID,
 		/* Snap to mid-point and adjacent edges. */
 		SNAP_MIDPOINT_AND_ADJACENT,
 		/* Snap to either bigger or smaller, nothing in-between (used for
@@ -1323,7 +1325,7 @@ static int area_move_init(bContext *C, wmOperator *op)
 	                     &md->bigger, &md->smaller,
 	                     &use_bigger_smaller_snap);
 
-	md->snap_type = use_bigger_smaller_snap ? SNAP_BIGGER_SMALLER_ONLY : SNAP_NONE;
+	md->snap_type = use_bigger_smaller_snap ? SNAP_BIGGER_SMALLER_ONLY : SNAP_AREAGRID;
 
 	return 1;
 }
@@ -1334,30 +1336,43 @@ static int area_snap_calc_location(
         const int bigger, const int smaller)
 {
 	BLI_assert(snap_type != SNAP_NONE);
-	if (snap_type == SNAP_BIGGER_SMALLER_ONLY) {
-		return ((origval + delta) >= bigger) ? bigger : smaller;
-	}
-
 	int final_loc = -1;
 	const int m_loc = origval + delta;
-	const int axis = (dir == 'v') ? 0 : 1;
-	int snap_dist;
-	int dist;
-	{
-		/* Test the snap to middle. */
-		int middle = origval + (bigger - smaller) / 2;
-		middle -= (middle % AREAGRID);
 
-		snap_dist = abs(m_loc - middle);
-		final_loc = middle;
-	}
+	switch (snap_type) {
+		case SNAP_AREAGRID:
+			final_loc = m_loc;
+			if (delta != bigger && delta != -smaller) {
+				final_loc -= (m_loc % AREAGRID);
+			}
+			break;
 
-	for (const ScrVert *v1 = sc->vertbase.first; v1; v1 = v1->next) {
-		if (v1->editflag) {
-			const int v_loc = (&v1->vec.x)[!axis];
+		case SNAP_BIGGER_SMALLER_ONLY:
+			final_loc = (m_loc >= bigger) ? bigger : smaller;
+			break;
+
+		case SNAP_MIDPOINT_AND_ADJACENT:
+		{
+			const int axis = (dir == 'v') ? 0 : 1;
+			int snap_dist;
+			int dist;
+			{
+				/* Test the snap to middle. */
+				int middle = origval + (bigger - smaller) / 2;
+				snap_dist = abs(m_loc - middle);
+				final_loc = middle;
+			}
 
-			for (const ScrVert *v2 = sc->vertbase.first; v2; v2 = v2->next) {
-				if (!v2->editflag) {
+			for (const ScrVert *v1 = sc->vertbase.first; v1; v1 = v1->next) {
+				if (!v1->editflag) {
+					continue;
+				}
+				const int v_loc = (&v1->vec.x)[!axis];
+
+				for (const ScrVert *v2 = sc->vertbase.first; v2; v2 = v2->next) {
+					if (v2->editflag) {
+						continue;
+					}
 					if (v_loc == (&v2->vec.x)[!axis]) {
 						const int v_loc2 = (&v2->vec.x)[axis];
 						/* Do not snap to the vertices at the ends. */
@@ -1371,6 +1386,7 @@ static int area_snap_calc_location(
 					}
 				}
 			}
+			break;
 		}
 	}
 
@@ -1393,9 +1409,6 @@ static void area_move_apply_do(
 
 	if (snap_type == SNAP_NONE) {
 		final_loc = origval + delta;
-		if (delta != bigger && delta != -smaller) {
-			final_loc -= (final_loc % AREAGRID);
-		}
 	}
 	else {
 		final_loc = area_snap_calc_location(sc, snap_type, delta, origval, dir, bigger, smaller);
@@ -1526,13 +1539,14 @@ static int area_move_modal(bContext *C, wmOperator *op, const wmEvent *event)
 					return OPERATOR_CANCELLED;
 
 				case KM_MODAL_SNAP_ON:
-					if (md->snap_type == SNAP_NONE) {
+					if (md->snap_type != SNAP_BIGGER_SMALLER_ONLY) {
 						md->snap_type = SNAP_MIDPOINT_AND_ADJACENT;
 					}
 					break;
+
 				case KM_MODAL_SNAP_OFF:
-					if (md->snap_type == SNAP_MIDPOINT_AND_ADJACENT) {
-						md->snap_type = SNAP_NONE;
+					if (md->snap_type != SNAP_BIGGER_SMALLER_ONLY) {
+						md->snap_type = SNAP_AREAGRID;
 					}
 					break;
 			}
@@ -1652,7 +1666,7 @@ static int area_split_init(bContext *C, wmOperator *op)
 {
 	ScrArea *sa = CTX_wm_area(C);
 	sAreaSplitData *sd;
-	int areaminy = ED_area_headersize() + 1;
+	int areaminy = ED_area_headersize();
 	int dir;
 	
 	/* required context */
@@ -1670,9 +1684,15 @@ static int area_split_init(bContext *C, wmOperator *op)
 	op->customdata = sd;
 	
 	sd->sarea = sa;
-	sd->origsize = dir == 'v' ? sa->winx : sa->winy;
-	sd->origmin = dir == 'v' ? sa->totrct.xmin : sa->totrct.ymin;
-	
+	if (dir == 'v') {
+		sd->origmin = sa->v1->vec.x;
+		sd->origsize = sa->v4->vec.x - sd->origmin;
+	}
+	else {
+		sd->origmin = sa->v1->vec.y;
+		sd->origsize = sa->v2->vec.y - sd->origmin;
+	}
+
 	return 1;
 }
 
@@ -1991,12 +2011,12 @@ static int area_split_modal(bContext *C, wmOperator *op, const wmEvent *event)
 			if (sd->sarea) {
 				ScrArea *sa = sd->sarea;
 				if (dir == 'v') {
-					sd->origsize = sa->winx;
-					sd->origmin = sa->totrct.xmin;
+					sd->origmin = sa->v1->vec.x;
+					sd->origsize = sa->v4->vec.x - sd->origmin;
 				}
 				else {
-					sd->origsize = sa->winy;
-					sd->origmin = sa->totrct.ymin;
+					sd->origmin = sa->v1->vec.y;
+					sd->origsize = sa->v2->vec.y - sd->origmin;
 				}
 
 				if (sd->do_snap) {