Avoid crashing on too many containers

If far too many containers are created, they can become so small that
their size calculations come out negative, leading to crashes on
asserts.

Instead, set a lower bound for sizes and disable the container entirely
if it goes below it, giving whatever space it used to the last
container.

The splits are not recalculated, so currently the effect is that if all
containers have the same width fraction, they keep getting narrower
until at some point they all round to zero and the last container will
be given all the available space.

A better behavior would have been if the additional container did not
contribute to size and fraction calculations at all, but it's an extreme
edge-case, anything is better than crashing, and this is easier to
implement.

Author: kennylevinsen

sway/desktop/transaction.c

@@ -312,9 +312,9 @@ static void arrange_children(enum sway_container_layout layout, list_t *children
 			wlr_scene_node_set_position(&child->scene_tree->node, 0, title_bar_height);
 			wlr_scene_node_reparent(&child->scene_tree->node, content);
 
-			if (activated) {
-				arrange_container(child, width, height - title_bar_height,
-					title_bar_height == 0, 0);
+			height -= title_bar_height;
+			if (activated && width > 0 && height > 0) {
+				arrange_container(child, width, height, title_bar_height == 0, 0);
 			} else {
 				disable_container(child);
 			}
@@ -341,9 +341,9 @@ static void arrange_children(enum sway_container_layout layout, list_t *children
 			wlr_scene_node_set_position(&child->scene_tree->node, 0, title_height);
 			wlr_scene_node_reparent(&child->scene_tree->node, content);
 
-			if (activated) {
-				arrange_container(child, width, height - title_height,
-					title_bar_height == 0, 0);
+			height -= title_bar_height;
+			if (activated && width > 0 && height > 0) {
+				arrange_container(child, width, height, title_bar_height == 0, 0);
 			} else {
 				disable_container(child);
 			}
@@ -359,8 +359,12 @@ static void arrange_children(enum sway_container_layout layout, list_t *children
 			wlr_scene_node_set_enabled(&child->border.tree->node, true);
 			wlr_scene_node_set_position(&child->scene_tree->node, 0, off);
 			wlr_scene_node_reparent(&child->scene_tree->node, content);
-			arrange_container(child, width, cheight, true, gaps);
-			off += cheight + gaps;
+			if (width > 0 && cheight > 0) {
+				arrange_container(child, width, cheight, true, gaps);
+				off += cheight + gaps;
+			} else {
+				disable_container(child);
+			}
 		}
 	} else if (layout == L_HORIZ) {
 		int off = 0;
@@ -372,7 +376,12 @@ static void arrange_children(enum sway_container_layout layout, list_t *children
 			wlr_scene_node_set_position(&child->scene_tree->node, off, 0);
 			wlr_scene_node_reparent(&child->scene_tree->node, content);
 			arrange_container(child, cwidth, height, true, gaps);
-			off += cwidth + gaps;
+			if (cwidth > 0 && height > 0) {
+				arrange_container(child, cwidth, height, true, gaps);
+				off += cwidth + gaps;
+			} else {
+				disable_container(child);
+			}
 		}
 	} else {
 		sway_assert(false, "unreachable");

sway/tree/arrange.c

@@ -29,7 +29,7 @@ static void apply_horiz_layout(list_t *children, struct wlr_box *parent) {
 		}
 	}
 
-	// Calculate each height fraction
+	// Calculate each width fraction
 	double total_width_fraction = 0;
 	for (int i = 0; i < children->length; ++i) {
 		struct sway_container *child = children->items[i];
@@ -82,12 +82,18 @@ static void apply_horiz_layout(list_t *children, struct wlr_box *parent) {
 		child->pending.y = parent->y;
 		child->pending.width = round(child->width_fraction * child_total_width);
 		child->pending.height = parent->height;
-		child_x += child->pending.width + inner_gap;
 
 		// Make last child use remaining width of parent
 		if (i == children->length - 1) {
 			child->pending.width = parent->x + parent->width - child->pending.x;
 		}
+
+		// Arbitrary lower bound for window size
+		if (child->pending.width < 10 || child->pending.height < 10) {
+			child->pending.width = 0;
+			child->pending.height = 0;
+		}
+		child_x += child->pending.width + inner_gap;
 	}
 }
 
@@ -161,12 +167,18 @@ static void apply_vert_layout(list_t *children, struct wlr_box *parent) {
 		child->pending.y = child_y;
 		child->pending.width = parent->width;
 		child->pending.height = round(child->height_fraction * child_total_height);
-		child_y += child->pending.height + inner_gap;
 
 		// Make last child use remaining height of parent
 		if (i == children->length - 1) {
 			child->pending.height = parent->y + parent->height - child->pending.y;
 		}
+
+		// Arbitrary lower bound for window size
+		if (child->pending.width < 10 || child->pending.height < 10) {
+			child->pending.width = 0;
+			child->pending.height = 0;
+		}
+		child_y += child->pending.height + inner_gap;
 	}
 }