Relayout terminals when they are set to visible

This fixes an issue where background terminals were created while not
visible. This caused the internal charMeasure object to have 0 width and
height so nothing would render.

Fixes #34554

src/vs/workbench/parts/terminal/electron-browser/terminalInstance.ts

@@ -511,6 +511,14 @@ export class TerminalInstance implements ITerminalInstance {
 			// background since scrollTop changes take no effect but the terminal's position does
 			// change since the number of visible rows decreases.
 			this._xterm.emit('scroll', this._xterm.buffer.ydisp);
+			// Force a layout when the instance becomes invisible. This is particularly important
+			// for ensuring that terminals that are created in the background by an extension will
+			// correctly get correct character measurements in order to render to the screen (see
+			// #34554).
+			const computedStyle = window.getComputedStyle(this._container);
+			const width = parseInt(computedStyle.getPropertyValue('width').replace('px', ''), 10);
+			const height = parseInt(computedStyle.getPropertyValue('height').replace('px', ''), 10);
+			this.layout(new Dimension(width, height));
 		}
 	}