Fix pixel leaks when using fractional scaling

The basic idea here is to apply rounding after scaling. It's not as
simple as this, though, and I've detailed it in the comments for a
function.

In order to fix some pixel leaks in the title bar, I found it easier to
change how we place rectangles to fill the area. Instead of placing two
rectangles across the full width above and below the title and having
shorter rectangles in the inner area, it's now pieced together in
vertical chunks. This method involves drawing two less rectangles per
container.

2 files changed, 89 insertions, 88 deletions

diff --git a/sway/desktop/output.c b/sway/desktop/output.c
index adc1ee10..fc52dd28 100644
--- a/sway/desktop/output.c
+++ b/sway/desktop/output.c
@@ -223,11 +223,15 @@ void output_drag_icons_for_each_surface(struct sway_output *output,
 	}
 }
 
+static int scale_length(int length, int offset, float scale) {
+	return round((offset + length) * scale) - round(offset * scale);
+}
+
 static void scale_box(struct wlr_box *box, float scale) {
-	box->x *= scale;
-	box->y *= scale;
-	box->width *= scale;
-	box->height *= scale;
+	box->width = scale_length(box->width, box->x, scale);
+	box->height = scale_length(box->height, box->y, scale);
+	box->x = round(box->x * scale);
+	box->y = round(box->y * scale);
 }
 
 struct sway_workspace *output_get_active_workspace(struct sway_output *output) {
diff --git a/sway/desktop/render.c b/sway/desktop/render.c
index 3617da87..9b26c560 100644
--- a/sway/desktop/render.c
+++ b/sway/desktop/render.c
@@ -33,11 +33,27 @@ struct render_data {
 	float alpha;
 };
 
+/**
+ * Apply scale to a width or height.
+ *
+ * One does not simply multiply the width by the scale. We allow fractional
+ * scaling, which means the resulting scaled width might be a decimal.
+ * So we round it.
+ *
+ * But even this can produce undesirable results depending on the X or Y offset
+ * of the box. For example, with a scale of 1.5, a box with width=1 should not
+ * scale to 2px if its X coordinate is 1, because the X coordinate would have
+ * scaled to 2px.
+ */
+static int scale_length(int length, int offset, float scale) {
+	return round((offset + length) * scale) - round(offset * scale);
+}
+
 static void scale_box(struct wlr_box *box, float scale) {
-	box->x *= scale;
-	box->y *= scale;
-	box->width *= scale;
-	box->height *= scale;
+	box->width = scale_length(box->width, box->x, scale);
+	box->height = scale_length(box->height, box->y, scale);
+	box->x = round(box->x * scale);
+	box->y = round(box->y * scale);
 }
 
 static void scissor_output(struct wlr_output *wlr_output,
@@ -392,14 +408,23 @@ static void render_titlebar(struct sway_output *output,
 		render_rect(output->wlr_output, output_damage, &box, color);
 
 		// Single pixel right edge
-		box.x = (x + width - TITLEBAR_BORDER_THICKNESS) * output_scale;
+		box.x = x + width - TITLEBAR_BORDER_THICKNESS;
+		box.y = y + TITLEBAR_BORDER_THICKNESS;
+		box.width = TITLEBAR_BORDER_THICKNESS;
+		box.height =
+			container_titlebar_height() - TITLEBAR_BORDER_THICKNESS * 2;
+		scale_box(&box, output_scale);
 		render_rect(output->wlr_output, output_damage, &box, color);
 	}
 
 	size_t inner_width = width - TITLEBAR_H_PADDING * 2;
+	int bg_y = y + TITLEBAR_BORDER_THICKNESS;
+	int ob_bg_height = scale_length(
+			(TITLEBAR_V_PADDING - TITLEBAR_BORDER_THICKNESS) * 2 +
+			config->font_height, bg_y, output_scale);
 
 	// Marks
-	size_t marks_ob_width = 0; // output-buffer-local
+	int marks_ob_width = 0; // output-buffer-local
 	if (config->show_marks && marks_texture) {
 		struct wlr_box texture_box;
 		wlr_texture_get_size(marks_texture,
@@ -408,15 +433,14 @@ static void render_titlebar(struct sway_output *output,
 
 		// 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);
+		int ob_padding_total = ob_bg_height - texture_box.height;
+		int ob_padding_above = floor(ob_padding_total / 2.0);
+		int ob_padding_below = ceil(ob_padding_total / 2.0);
 
 		// 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.x = round((x - output_x + width - TITLEBAR_H_PADDING)
+				* output_scale) - texture_box.width;
+		texture_box.y = round((bg_y - output_y) * output_scale) +
 			ob_padding_above;
 
 		float matrix[9];
@@ -431,29 +455,18 @@ static void render_titlebar(struct sway_output *output,
 			&texture_box, matrix, con->alpha);
 
 		// Padding above
-		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);
-		}
+		memcpy(&color, colors->background, sizeof(float) * 4);
+		premultiply_alpha(color, con->alpha);
+		box.x = texture_box.x + round(output_x * output_scale);
+		box.y = round((y + TITLEBAR_BORDER_THICKNESS) * 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 = (x + width - TITLEBAR_H_PADDING) * output_scale -
-				texture_box.width;
-			box.y = (y + TITLEBAR_V_PADDING) * output_scale +
-				ob_padding_above + texture_box.height;
-			box.width = texture_box.width;
-			box.height = ob_padding_below;
-			render_rect(output->wlr_output, output_damage, &box, color);
-		}
+		box.y += ob_padding_above + texture_box.height;
+		box.height = ob_padding_below;
+		render_rect(output->wlr_output, output_damage, &box, color);
 	}
 
 	// Title text
@@ -466,89 +479,73 @@ static void render_titlebar(struct sway_output *output,
 
 		// The title texture might be shorter than the config->font_height,
 		// in which case we need to pad it above and below.
-		int ob_padding_above = (config->font_baseline - con->title_baseline)
-			* output_scale;
-		int ob_padding_below = (config->font_height - con->title_height)
-			* output_scale - ob_padding_above;
+		int ob_padding_above = round((config->font_baseline -
+					con->title_baseline + TITLEBAR_V_PADDING -
+					TITLEBAR_BORDER_THICKNESS) * output_scale);
+		int ob_padding_below = ob_bg_height - ob_padding_above -
+			texture_box.height;
 
 		// Render texture
-		texture_box.x = (x - output_x + TITLEBAR_H_PADDING) * output_scale;
-		texture_box.y = (y - output_y + TITLEBAR_V_PADDING) * output_scale +
-			ob_padding_above;
+		texture_box.x =
+			round((x - output_x + TITLEBAR_H_PADDING) * output_scale);
+		texture_box.y =
+			round((bg_y - output_y) * output_scale) + ob_padding_above;
 
 		float matrix[9];
 		wlr_matrix_project_box(matrix, &texture_box,
 			WL_OUTPUT_TRANSFORM_NORMAL,
 			0.0, output->wlr_output->transform_matrix);
 
-		if (inner_width * output_scale - marks_ob_width < texture_box.width) {
-			texture_box.width = inner_width * output_scale - marks_ob_width;
+		int inner_x = x - output_x + TITLEBAR_H_PADDING;
+		int ob_inner_width = scale_length(inner_width, inner_x, output_scale);
+		if (ob_inner_width - marks_ob_width < texture_box.width) {
+			texture_box.width = ob_inner_width - marks_ob_width;
 		}
 		render_texture(output->wlr_output, output_damage, title_texture,
 			&texture_box, matrix, con->alpha);
 
 		// Padding above
-		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);
-		}
+		memcpy(&color, colors->background, sizeof(float) * 4);
+		premultiply_alpha(color, con->alpha);
+		box.x = texture_box.x + round(output_x * output_scale);
+		box.y = round((y + TITLEBAR_BORDER_THICKNESS) * 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 = (x + TITLEBAR_H_PADDING) * output_scale;
-			box.y = (y + TITLEBAR_V_PADDING) * output_scale +
-				ob_padding_above + texture_box.height;
-			box.width = texture_box.width;
-			box.height = ob_padding_below;
-			render_rect(output->wlr_output, output_damage, &box, color);
-		}
+		box.y += ob_padding_above + texture_box.height;
+		box.height = ob_padding_below;
+		render_rect(output->wlr_output, output_damage, &box, color);
 	}
 
-	// Padding above title
-	memcpy(&color, colors->background, sizeof(float) * 4);
-	premultiply_alpha(color, con->alpha);
-	box.x = x + (layout == L_TABBED) * TITLEBAR_BORDER_THICKNESS;
-	box.y = y + TITLEBAR_BORDER_THICKNESS;
-	box.width = width - (layout == L_TABBED) * TITLEBAR_BORDER_THICKNESS * 2;
-	box.height = TITLEBAR_V_PADDING - TITLEBAR_BORDER_THICKNESS;
-	scale_box(&box, output_scale);
-	render_rect(output->wlr_output, output_damage, &box, color);
-
-	// Padding below title
-	box.y = (y + TITLEBAR_V_PADDING + config->font_height) * output_scale;
-	render_rect(output->wlr_output, output_damage, &box, color);
-
 	// Filler between title and marks
-	box.width = inner_width * output_scale - title_ob_width - marks_ob_width;
+	box.width =
+		round(inner_width * output_scale) - title_ob_width - marks_ob_width;
 	if (box.width > 0) {
-		box.x = (x + TITLEBAR_H_PADDING) * output_scale + title_ob_width;
-		box.y = (y + TITLEBAR_V_PADDING) * output_scale;
-		box.height = config->font_height * output_scale;
+		box.x = round((x + TITLEBAR_H_PADDING) * output_scale) + title_ob_width;
+		box.y = round(bg_y * output_scale);
+		box.height = ob_bg_height;
 		render_rect(output->wlr_output, output_damage, &box, color);
 	}
 
 	// Padding left of title
 	left_offset = (layout == L_TABBED) * TITLEBAR_BORDER_THICKNESS;
 	box.x = x + left_offset;
-	box.y = y + TITLEBAR_V_PADDING;
+	box.y = y + TITLEBAR_BORDER_THICKNESS;
 	box.width = TITLEBAR_H_PADDING - left_offset;
-	box.height = config->font_height;
+	box.height = (TITLEBAR_V_PADDING - TITLEBAR_BORDER_THICKNESS) * 2 +
+		config->font_height;
 	scale_box(&box, output_scale);
 	render_rect(output->wlr_output, output_damage, &box, color);
 
 	// Padding right of marks
 	right_offset = (layout == L_TABBED) * TITLEBAR_BORDER_THICKNESS;
 	box.x = x + width - TITLEBAR_H_PADDING;
-	box.y = y + TITLEBAR_V_PADDING;
+	box.y = y + TITLEBAR_BORDER_THICKNESS;
 	box.width = TITLEBAR_H_PADDING - right_offset;
-	box.height = config->font_height;
+	box.height = (TITLEBAR_V_PADDING - TITLEBAR_BORDER_THICKNESS) * 2 +
+		config->font_height;
 	scale_box(&box, output_scale);
 	render_rect(output->wlr_output, output_damage, &box, color);