Rework gaps code to be simpler and correct

Instead of tracking gaps per child apply gaps in two logical places:

1. In tiled containers use the layout code to add the gaps between
windows. This is much simpler and guarantees that the sizing of children
is correct.
2. In the workspace itself apply all the gaps around the edge. Here
we're in the correct position to size inner and outer gaps correctly and
decide on smart gaps in a single location.

Fixes #4296

sway/tree/arrange.c

@@ -52,23 +52,39 @@ static void apply_horiz_layout(list_t *children, struct wlr_box *parent) {
 		child->width_fraction /= total_width_fraction;
 	}
 
+	// Calculate gap size
+	double inner_gap = 0;
+	struct sway_container *child = children->items[0];
+	struct sway_workspace *ws = child->workspace;
+	if (ws) {
+		inner_gap = ws->gaps_inner;
+	}
+	// Descendants of tabbed/stacked containers don't have gaps
+	struct sway_container *temp = child;
+	while (temp) {
+		enum sway_container_layout layout = container_parent_layout(temp);
+		if (layout == L_TABBED || layout == L_STACKED) {
+			inner_gap = 0;
+		}
+		temp = temp->parent;
+	}
+	double child_total_width = parent->width - inner_gap * (children->length - 1);
+
 	// Resize windows
 	sway_log(SWAY_DEBUG, "Arranging %p horizontally", parent);
 	double child_x = parent->x;
 	for (int i = 0; i < children->length; ++i) {
 		struct sway_container *child = children->items[i];
-		container_remove_gaps(child);
 		child->x = child_x;
 		child->y = parent->y;
-		child->width = floor(child->width_fraction * parent->width);
+		child->width = floor(child->width_fraction * child_total_width);
 		child->height = parent->height;
-		child_x += child->width;
+		child_x += child->width + inner_gap;
 
 		// Make last child use remaining width of parent
 		if (i == children->length - 1) {
 			child->width = parent->x + parent->width - child->x;
 		}
-		container_add_gaps(child);
 	}
 }
 
@@ -111,23 +127,39 @@ static void apply_vert_layout(list_t *children, struct wlr_box *parent) {
 		child->height_fraction /= total_height_fraction;
 	}
 
-	// Resize
+	// Calculate gap size
+	double inner_gap = 0;
+	struct sway_container *child = children->items[0];
+	struct sway_workspace *ws = child->workspace;
+	if (ws) {
+		inner_gap = ws->gaps_inner;
+	}
+	// Descendants of tabbed/stacked containers don't have gaps
+	struct sway_container *temp = child;
+	while (temp) {
+		enum sway_container_layout layout = container_parent_layout(temp);
+		if (layout == L_TABBED || layout == L_STACKED) {
+			inner_gap = 0;
+		}
+		temp = temp->parent;
+	}
+	double child_total_height = parent->height - inner_gap * (children->length - 1);
+
+	// Resize windows
 	sway_log(SWAY_DEBUG, "Arranging %p vertically", parent);
 	double child_y = parent->y;
 	for (int i = 0; i < children->length; ++i) {
 		struct sway_container *child = children->items[i];
-		container_remove_gaps(child);
 		child->x = parent->x;
 		child->y = child_y;
 		child->width = parent->width;
-		child->height = floor(child->height_fraction * parent->height);
-		child_y += child->height;
+		child->height = floor(child->height_fraction * child_total_height);
+		child_y += child->height + inner_gap;
 
 		// Make last child use remaining height of parent
 		if (i == children->length - 1) {
 			child->height = parent->y + parent->height - child->y;
 		}
-		container_add_gaps(child);
 	}
 }
 
@@ -138,12 +170,10 @@ static void apply_tabbed_layout(list_t *children, struct wlr_box *parent) {
 	for (int i = 0; i < children->length; ++i) {
 		struct sway_container *child = children->items[i];
 		int parent_offset = child->view ? 0 : container_titlebar_height();
-		container_remove_gaps(child);
 		child->x = parent->x;
 		child->y = parent->y + parent_offset;
 		child->width = parent->width;
 		child->height = parent->height - parent_offset;
-		container_add_gaps(child);
 	}
 }
 
@@ -155,12 +185,10 @@ static void apply_stacked_layout(list_t *children, struct wlr_box *parent) {
 		struct sway_container *child = children->items[i];
 		int parent_offset = child->view ?  0 :
 			container_titlebar_height() * children->length;
-		container_remove_gaps(child);
 		child->x = parent->x;
 		child->y = parent->y + parent_offset;
 		child->width = parent->width;
 		child->height = parent->height - parent_offset;
-		container_add_gaps(child);
 	}
 }
 
@@ -226,7 +254,6 @@ void arrange_workspace(struct sway_workspace *workspace) {
 	struct wlr_box *area = &output->usable_area;
 	sway_log(SWAY_DEBUG, "Usable area for ws: %dx%d@%d,%d",
 			area->width, area->height, area->x, area->y);
-	workspace_remove_gaps(workspace);
 
 	bool first_arrange = workspace->width == 0 && workspace->height == 0;
 	double prev_x = workspace->x;