Sync scroll bar after panel is toggled

Fixes #12574

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

@@ -342,6 +342,12 @@ export class TerminalInstance implements ITerminalInstance {
 		}
 		if (!dimension.height) { // Minimized
 			return;
+		} else {
+			// Trigger scroll event manually so that the viewport's scroll area is synced. This
+			// needs to happen otherwise its scrollTop value is invalid when the panel is toggled as
+			// it gets removed and then added back to the DOM (resetting scrollTop to 0).
+			// Upstream issue: https://github.com/sourcelair/xterm.js/issues/291
+			this._xterm.emit('scroll', this._xterm.ydisp);
 		}
 		let leftPadding = parseInt(getComputedStyle(document.querySelector('.terminal-outer-container')).paddingLeft.split('px')[0], 10);
 		let innerWidth = dimension.width - leftPadding;