Fix gaps in title textures and vertically center them

There was code that attempted to fill in the gap below the title texture
when the texture isn't tall enough, but this only worked when the output
was positioned at 0,0. The reason is that render_rect expects a box
passed in a hybrid layout-local/output-buffer-local system, and we were
passing purely output-buffer-local. I've added a comment documenting
this.

By the way, we can't use layout-local coordinates for the rectangle box
because in some cases we set the box based on a texture size. Texture
sizes are buffer-local, and we'd have to divide them to bring it back to
layout-local which means losing precision. We could use
output-buffer-local coordinates for the box, but this would require
translating the coordinates from layout-local to output-buffer-local in
many places during rendering.

This patch also vertically centers the text inside the title bar.

sway/desktop/render.c

@@ -146,6 +146,8 @@ static void render_drag_icons(struct sway_output *output,
 		render_surface_iterator, &data);
 }
 
+// _box.x and .y are expected to be layout-local
+// _box.width and .height are expected to be output-buffer-local
 static void render_rect(struct wlr_output *wlr_output,
 		pixman_region32_t *output_damage, const struct wlr_box *_box,
 		float color[static 4]) {
@@ -404,9 +406,20 @@ static void render_titlebar(struct sway_output *output,
 		struct wlr_box texture_box;
 		wlr_texture_get_size(marks_texture,
 			&texture_box.width, &texture_box.height);
+		marks_ob_width = texture_box.width;
+
+		// The marks texture might be shorter than the config->font_height, in
+		// which case we need to pad it as evenly as possible above and below.
+		int ob_padding_total = config->font_height * output_scale -
+			texture_box.height;
+		int ob_padding_above = floor(ob_padding_total / 2);
+		int ob_padding_below = ceil(ob_padding_total / 2);
+
+		// Render texture
 		texture_box.x = (x - output_x + width - TITLEBAR_H_PADDING)
 			* output_scale - texture_box.width;
-		texture_box.y = (y - output_y + TITLEBAR_V_PADDING) * output_scale;
+		texture_box.y = (y - output_y + TITLEBAR_V_PADDING) * output_scale +
+			ob_padding_above;
 
 		float matrix[9];
 		wlr_matrix_project_box(matrix, &texture_box,
@@ -418,17 +431,29 @@ static void render_titlebar(struct sway_output *output,
 		}
 		render_texture(output->wlr_output, output_damage, marks_texture,
 			&texture_box, matrix, con->alpha);
-		marks_ob_width = texture_box.width;
 
-		// Gap between the marks and bottom padding, for when the marks texture
+		// Padding above
-		// height is smaller than the config's font height
+		if (ob_padding_above > 0) {
+			memcpy(&color, colors->background, sizeof(float) * 4);
+			premultiply_alpha(color, con->alpha);
+			box.x = (x + width - TITLEBAR_H_PADDING) * output_scale -
+				texture_box.width;
+			box.y = (y + TITLEBAR_V_PADDING) * output_scale;
+			box.width = texture_box.width;
+			box.height = ob_padding_above;
+			render_rect(output->wlr_output, output_damage, &box, color);
+		}
+
+		// Padding below
+		if (ob_padding_below > 0) {
 			memcpy(&color, colors->background, sizeof(float) * 4);
 			premultiply_alpha(color, con->alpha);
-		box.x = texture_box.x;
+			box.x = (x + width - TITLEBAR_H_PADDING) * output_scale -
-		box.y = texture_box.y + texture_box.height;
+				texture_box.width;
+			box.y = (y + TITLEBAR_V_PADDING) * output_scale +
+				ob_padding_above + texture_box.height;
 			box.width = texture_box.width;
-		box.height = config->font_height * output_scale - texture_box.height;
+			box.height = ob_padding_below;
-		if (box.height > 0) {
 			render_rect(output->wlr_output, output_damage, &box, color);
 		}
 	}
@@ -439,8 +464,19 @@ static void render_titlebar(struct sway_output *output,
 		struct wlr_box texture_box;
 		wlr_texture_get_size(title_texture,
 			&texture_box.width, &texture_box.height);
+		title_ob_width = texture_box.width;
+
+		// The title texture might be shorter than the config->font_height, in
+		// which case we need to pad it as evenly as possible above and below.
+		int ob_padding_total = config->font_height * output_scale -
+			texture_box.height;
+		int ob_padding_above = floor(ob_padding_total / 2);
+		int ob_padding_below = ceil(ob_padding_total / 2);
+
+		// Render texture
 		texture_box.x = (x - output_x + TITLEBAR_H_PADDING) * output_scale;
-		texture_box.y = (y - output_y + TITLEBAR_V_PADDING) * output_scale;
+		texture_box.y = (y - output_y + TITLEBAR_V_PADDING) * output_scale +
+			ob_padding_above;
 
 		float matrix[9];
 		wlr_matrix_project_box(matrix, &texture_box,
@@ -452,17 +488,27 @@ static void render_titlebar(struct sway_output *output,
 		}
 		render_texture(output->wlr_output, output_damage, title_texture,
 			&texture_box, matrix, con->alpha);
-		title_ob_width = texture_box.width;
 
-		// Gap between the title and bottom padding, for when the title texture
+		// Padding above
-		// height is smaller than the config's font height
+		if (ob_padding_above > 0) {
+			memcpy(&color, colors->background, sizeof(float) * 4);
+			premultiply_alpha(color, con->alpha);
+			box.x = (x + TITLEBAR_H_PADDING) * output_scale;
+			box.y = (y + TITLEBAR_V_PADDING) * output_scale;
+			box.width = texture_box.width;
+			box.height = ob_padding_above;
+			render_rect(output->wlr_output, output_damage, &box, color);
+		}
+
+		// Padding below
+		if (ob_padding_below > 0) {
 			memcpy(&color, colors->background, sizeof(float) * 4);
 			premultiply_alpha(color, con->alpha);
-		box.x = texture_box.x;
+			box.x = (x + TITLEBAR_H_PADDING) * output_scale;
-		box.y = texture_box.y + texture_box.height;
+			box.y = (y + TITLEBAR_V_PADDING) * output_scale +
+				ob_padding_above + texture_box.height;
 			box.width = texture_box.width;
-		box.height = config->font_height * output_scale - texture_box.height;
+			box.height = ob_padding_below;
-		if (box.height > 0) {
 			render_rect(output->wlr_output, output_damage, &box, color);
 		}
 	}