fix: clear canvas before filling to support transparent backgrounds

When using a transparent background color, fillRect composites rather than
replaces pixels. Add clearRect before fillRect in renderLine, clear, and
renderScrollbar to properly erase previous content before applying the
background color. This fixes content piling up when using transparent
backgrounds and ensures selection highlighting clears properly.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: slang25

lib/renderer.ts

@@ -515,12 +515,16 @@ export class CanvasRenderer {
    */
   private renderLine(line: GhosttyCell[], y: number, cols: number): void {
     const lineY = y * this.metrics.height;
+    const lineWidth = cols * this.metrics.width;
 
-    // Clear line background with theme color.
+    // Clear line background then fill with theme color.
     // We clear just the cell area - glyph overflow is handled by also
     // redrawing adjacent rows (see render() method).
+    // clearRect is needed because fillRect composites rather than replaces,
+    // so transparent/translucent backgrounds wouldn't clear previous content.
+    this.ctx.clearRect(0, lineY, lineWidth, this.metrics.height);
     this.ctx.fillStyle = this.theme.background;
-    this.ctx.fillRect(0, lineY, cols * this.metrics.width, this.metrics.height);
+    this.ctx.fillRect(0, lineY, lineWidth, this.metrics.height);
 
     // PASS 1: Draw all cell backgrounds first
     // This ensures all backgrounds are painted before any text, allowing text
@@ -790,6 +794,7 @@ export class CanvasRenderer {
       this.theme.brightCyan,
       this.theme.brightWhite,
     ];
+
   }
 
   /**
@@ -854,6 +859,7 @@ export class CanvasRenderer {
     const scrollbarTrackHeight = canvasHeight - scrollbarPadding * 2;
 
     // Always clear the scrollbar area first (fixes ghosting when fading out)
+    ctx.clearRect(scrollbarX - 2, 0, scrollbarWidth + 6, canvasHeight);
     ctx.fillStyle = this.theme.background;
     ctx.fillRect(scrollbarX - 2, 0, scrollbarWidth + 6, canvasHeight);
 
@@ -966,6 +972,9 @@ export class CanvasRenderer {
    * Clear entire canvas
    */
   public clear(): void {
+    // clearRect first because fillRect composites rather than replaces,
+    // so transparent/translucent backgrounds wouldn't clear previous content.
+    this.ctx.clearRect(0, 0, this.canvas.width, this.canvas.height);
     this.ctx.fillStyle = this.theme.background;
     this.ctx.fillRect(0, 0, this.canvas.width, this.canvas.height);
   }