Add +1 to the visible textarea width to avoid flickering that might be caused by accumulating rounding errors (microsoft#141725)

Author: alexdima

src/vs/editor/browser/controller/textAreaHandler.ts

@@ -650,7 +650,14 @@ export class TextAreaHandler extends ViewPart {
 
 				let scrollLeft = this._visibleTextArea.widthOfHiddenLineTextBefore;
 				let left = (this._contentLeft + visibleStart.left - this._scrollLeft);
-				let width = visibleEnd.left - visibleStart.left;
+				// See https://github.com/microsoft/vscode/issues/141725#issuecomment-1050670841
+				// Here we are adding +1 to avoid flickering that might be caused by having a width that is too small.
+				// This could be caused by rounding errors that might only show up with certain font families.
+				// In other words, a pixel might be lost when doing something like
+				//      `Math.round(end) - Math.round(start)`
+				// vs
+				//      `Math.round(end - start)`
+				let width = visibleEnd.left - visibleStart.left + 1;
 				if (left < this._contentLeft) {
 					// the textarea would be rendered on top of the margin,
 					// so reduce its width. We use the same technique as