Fixes microsoft/monaco-editor#2175: Improve hit testing code on FF

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

@@ -940,12 +940,16 @@ export class MouseTargetFactory {
 			}
 		}
 
-		// For inline decorations, Gecko returns the `<span>` of the line and the offset is the `<span>` with the inline decoration
+		// For inline decorations, Gecko sometimes returns the `<span>` of the line and the offset is the `<span>` with the inline decoration
+		// Some other times, it returns the `<span>` with the inline decoration
 		if (hitResult.offsetNode.nodeType === hitResult.offsetNode.ELEMENT_NODE) {
-			const parent1 = hitResult.offsetNode.parentNode; // expected to be the view line div
+			const parent1 = hitResult.offsetNode.parentNode;
 			const parent1ClassName = parent1 && parent1.nodeType === parent1.ELEMENT_NODE ? (<HTMLElement>parent1).className : null;
+			const parent2 = parent1 ? parent1.parentNode : null;
+			const parent2ClassName = parent2 && parent2.nodeType === parent2.ELEMENT_NODE ? (<HTMLElement>parent2).className : null;
 
 			if (parent1ClassName === ViewLine.CLASS_NAME) {
+				// it returned the `<span>` of the line and the offset is the `<span>` with the inline decoration
 				const tokenSpan = hitResult.offsetNode.childNodes[Math.min(hitResult.offset, hitResult.offsetNode.childNodes.length - 1)];
 				if (tokenSpan) {
 					const p = ctx.getPositionFromDOMInfo(<HTMLElement>tokenSpan, 0);
@@ -954,6 +958,13 @@ export class MouseTargetFactory {
 						hitTarget: null
 					};
 				}
+			} else if (parent2ClassName === ViewLine.CLASS_NAME) {
+				// it returned the `<span>` with the inline decoration
+				const p = ctx.getPositionFromDOMInfo(<HTMLElement>hitResult.offsetNode, 0);
+				return {
+					position: p,
+					hitTarget: null
+				};
 			}
 		}