Fix T41427: Region overlap moves into wrong window.

There was some fuzzyness in `region_overlap_fix()`, using an 'other side' region
as ref to move current one in case their rect would intersect...

New code is a bit more complex, but should handle nicely all situations, mostly
ensuring we only translate an overlap if we find a previous one **on the same side**,
and ensuring we also never have intersecting overlapping regions from different sides
(since this does not work nice at all).

1 files changed, 38 insertions, 17 deletions

diff --git a/source/blender/editors/screen/area.c b/source/blender/editors/screen/area.c
index 583ecf7db43..05418d8d451 100644
--- a/source/blender/editors/screen/area.c
+++ b/source/blender/editors/screen/area.c
@@ -891,38 +891,59 @@ static int rct_fits(rcti *rect, char dir, int size)
 /* function checks if some overlapping region was defined before - on same place */
 static void region_overlap_fix(ScrArea *sa, ARegion *ar)
 {
-	ARegion *ar1 = ar->prev;
-	
+	ARegion *ar1;
+	const int align = ar->alignment & ~RGN_SPLIT_PREV;
+	int align1 = 0;
+
 	/* find overlapping previous region on same place */
-	while (ar1) {
-		if (ar1->overlap) {
-			if ((ar1->alignment & RGN_SPLIT_PREV) == 0)
-				if (BLI_rcti_isect(&ar1->winrct, &ar->winrct, NULL))
-					break;
+	for (ar1 = ar->prev; ar1; ar1 = ar1->prev) {
+		if (ar1->overlap && ((ar1->alignment & RGN_SPLIT_PREV) == 0)) {
+			align1 = ar1->alignment;
+			if (BLI_rcti_isect(&ar1->winrct, &ar->winrct, NULL)) {
+				if (align1 != align) {
+					/* Left overlapping right or vice-versa, forbid this! */
+					ar->flag |= RGN_FLAG_TOO_SMALL;
+					return;
+				}
+				/* Else, we have our previous region on same side. */
+				break;
+			}
 		}
-		ar1 = ar1->prev;
 	}
-	
+
 	/* translate or close */
 	if (ar1) {
-		int align1 = ar1->alignment & ~RGN_SPLIT_PREV;
-
 		if (align1 == RGN_ALIGN_LEFT) {
-			if (ar->winrct.xmax + ar1->winx > sa->winx - U.widget_unit)
+			if (ar->winrct.xmax + ar1->winx > sa->winx - U.widget_unit) {
 				ar->flag |= RGN_FLAG_TOO_SMALL;
-			else
+				return;
+			}
+			else {
 				BLI_rcti_translate(&ar->winrct, ar1->winx, 0);
+			}
 		}
 		else if (align1 == RGN_ALIGN_RIGHT) {
-			if (ar->winrct.xmin - ar1->winx < U.widget_unit)
+			if (ar->winrct.xmin - ar1->winx < U.widget_unit) {
 				ar->flag |= RGN_FLAG_TOO_SMALL;
-			else
+				return;
+			}
+			else {
 				BLI_rcti_translate(&ar->winrct, -ar1->winx, 0);
+			}
 		}
 	}
 
-	
-	
+	/* At this point, 'ar' is in its final position and still open.
+	 * Make a final check it does not overlap any previous 'other side' region. */
+	for (ar1 = ar->prev; ar1; ar1 = ar1->prev) {
+		if (ar1->overlap && (ar1->alignment & RGN_SPLIT_PREV) == 0) {
+			if ((ar1->alignment != align) && BLI_rcti_isect(&ar1->winrct, &ar->winrct, NULL)) {
+				/* Left overlapping right or vice-versa, forbid this! */
+				ar->flag |= RGN_FLAG_TOO_SMALL;
+				return;
+			}
+		}
+	}
 }
 
 /* overlapping regions only in the following restricted cases */