exposay: centralize surfaces of the last row when we don't have enough surfaces

The exposay grid is square, but we don't always have enough surfaces
to fill all the columns of the last row. The code to centralize
the surfaces of the last row is not working.

Fix the code that centralizes the surfaces in the last row, making
it more visually pleasant.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

desktop-shell/exposay.c

@@ -262,9 +262,9 @@ exposay_layout(struct desktop_shell *shell, struct shell_output *shell_output)
 	struct exposay_surface *esurface, *highlight = NULL;
 	pixman_rectangle32_t exposay_area;
 	int pad, row_size, column_size, left_margin, top_margin;
+	int last_row_size, last_row_margin_increase;
 	int populated_rows;
 	int i;
-	int last_row_removed = 0;
 
 	eoutput->num_surfaces = 0;
 	wl_list_for_each(view, &workspace->layer.view_list.link, layer_link.link) {
@@ -292,7 +292,6 @@ exposay_layout(struct desktop_shell *shell, struct shell_output *shell_output)
 	eoutput->grid_size = floor(sqrtf(eoutput->num_surfaces));
 	if (pow(eoutput->grid_size, 2) != eoutput->num_surfaces)
 		eoutput->grid_size++;
-	last_row_removed = pow(eoutput->grid_size, 2) - eoutput->num_surfaces;
 
 	/* Fixed outer padding of 10% the size of the screen */
 	eoutput->hpadding_outer = (exposay_area.width / 10);
@@ -309,6 +308,15 @@ exposay_layout(struct desktop_shell *shell, struct shell_output *shell_output)
 	populated_rows = ceil(eoutput->num_surfaces / (float) eoutput->grid_size);
 	column_size = (pad * populated_rows) - eoutput->padding_inner;
 
+	/* The last row size can be different, since it may have less surfaces
+	 * than the grid size. Also, its margin may be increased to centralize
+	 * its surfaces, in the case where we don't have a perfect grid. */
+	last_row_size = ((eoutput->num_surfaces % eoutput->grid_size) * pad) - eoutput->padding_inner;
+	if (eoutput->num_surfaces % eoutput->grid_size)
+		last_row_margin_increase = (row_size - last_row_size) / 2;
+	else
+		last_row_margin_increase = 0;
+
 	/* Compute a top/left margin to centralize the exposay */
 	exposay_margin_size(shell, exposay_area, row_size, column_size, &left_margin, &top_margin);
 
@@ -338,8 +346,10 @@ exposay_layout(struct desktop_shell *shell, struct shell_output *shell_output)
 		esurface->x = left_margin + (pad * esurface->column);
 		esurface->y = top_margin + (pad * esurface->row);
 
-		if (esurface->row == eoutput->grid_size - 1)
+		/* If this is the last row, increase left margin (it sums 0 if
-			esurface->x += (eoutput->surface_size + eoutput->padding_inner) * last_row_removed / 2;
+		 * we have a perfect square) to centralize the surfaces */
+		if (eoutput->num_surfaces / eoutput->grid_size == esurface->row)
+			esurface->x += last_row_margin_increase;
 
 		if (view->surface->width > view->surface->height)
 			esurface->scale = eoutput->surface_size / (float) view->surface->width;