Fix new UI align code behavior with very small buttons.

In case two neighbor buttons are very small, their total width (or height) can remain below
max authorized delta, and hence wrong side could be detected as 'common side' for the pair.

This is now fixed by checking both opposite sides at once.

Also, we expect buttons to have some width and height to be considered alignable now!

Took me two days to sort that out, grrrr!

1 files changed, 17 insertions, 6 deletions

diff --git a/source/blender/editors/interface/interface_align.c b/source/blender/editors/interface/interface_align.c
index e1e4f22990d..89a44db5fa3 100644
--- a/source/blender/editors/interface/interface_align.c
+++ b/source/blender/editors/interface/interface_align.c
@@ -33,6 +33,7 @@
 
 #include "BLI_alloca.h"
 #include "BLI_math.h"
+#include "BLI_rect.h"
 
 #include "UI_interface.h"
 
@@ -109,7 +110,9 @@ enum {
 
 bool ui_but_can_align(const uiBut *but)
 {
-	return !ELEM(but->type, UI_BTYPE_LABEL, UI_BTYPE_CHECKBOX, UI_BTYPE_CHECKBOX_N, UI_BTYPE_SEPR, UI_BTYPE_SEPR_LINE);
+	return (
+	    !ELEM(but->type, UI_BTYPE_LABEL, UI_BTYPE_CHECKBOX, UI_BTYPE_CHECKBOX_N, UI_BTYPE_SEPR, UI_BTYPE_SEPR_LINE) &&
+	    (BLI_rctf_size_x(&but->rect) > 0.0f) && (BLI_rctf_size_y(&but->rect) > 0.0f));
 }
 
 /**
@@ -122,8 +125,8 @@ static void block_align_proximity_compute(ButAlign *butal, ButAlign *butal_other
 {
 	/* That's the biggest gap between two borders to consider them 'alignable'. */
 	const float max_delta = MAX_DELTA;
-	float delta;
-	int side;
+	float delta, delta_side_opp;
+	int side, side_opp;
 
 	const bool butal_can_align = ui_but_can_align(butal->but);
 	const bool butal_other_can_align = ui_but_can_align(butal_other->but);
@@ -142,18 +145,26 @@ static void block_align_proximity_compute(ButAlign *butal, ButAlign *butal_other
 		return;
 	}
 
-	for (side = 0; side < TOTSIDES; side++) {
+	for (side = 0; side < RIGHT; side++) {
 		/* We are only interested in buttons which share a same line (LEFT/RIGHT sides) or column (TOP/DOWN sides). */
 		if (buts_share[IS_COLUMN(side)]) {
-			const int side_opp = OPPOSITE(side);
+			side_opp = OPPOSITE(side);
 
+			/* We check both opposite sides at once, because with very small buttons, delta could be below max_delta for
+			 * the wrong side (that is, in horizontal case, the total width of two buttons can be below max_delta). */
 			/* We rely on exact zero value here as an 'already processed' flag, so ensure we never actually
 			 * set a zero value at this stage. FLT_MIN is zero-enough for UI position computing. ;) */
 			delta = max_ff(fabsf(*butal->borders[side] - *butal_other->borders[side_opp]), FLT_MIN);
+			delta_side_opp = max_ff(fabsf(*butal->borders[side_opp] - *butal_other->borders[side]), FLT_MIN);
+			if (delta_side_opp < delta) {
+				SWAP(int, side, side_opp);
+				delta = delta_side_opp;
+			}
+
 			if (delta < max_delta) {
 				/* We are only interested in neighbors that are at least as close as already found ones. */
 				if (delta <= butal->dists[side]) {
-					if (delta <= butal->dists[side]) {
+					if (delta < butal->dists[side]) {
 						/* We found a closer neighbor.
 						 * If both buttons are alignable, we set them as each other neighbors.
 						 * Else, we have an unalignable one, we need to reset the others matching neighbor to NULL