Fix hidden sash in terminal with right panel and splits

Fixes #79226

src/vs/workbench/contrib/terminal/browser/terminalInstance.ts

@@ -421,7 +421,7 @@ export class TerminalInstance extends Disposable implements ITerminalInstance {
 		const bottom = parseInt(wrapperElementStyle.bottom!.split('px')[0], 10);
 
 		const innerWidth = width - marginLeft - marginRight;
-		const innerHeight = height - bottom;
+		const innerHeight = height - bottom - 1;
 
 		TerminalInstance._lastKnownCanvasDimensions = new dom.Dimension(innerWidth, innerHeight);
 		return TerminalInstance._lastKnownCanvasDimensions;