Improve compatibility with file: protocol

src/functions.js

@@ -79,10 +79,15 @@ function set_all_url_params(params, {replace_history_state=false}={}) {
 	// This is desired for upgrading backwards compatibility URLs;
 	// may not be desired for future cases.
 	const new_url = `${location.origin}${location.pathname}#${new_hash}`;
-	if (replace_history_state) {
-		history.replaceState(null, document.title, new_url);
-	} else {
-		history.pushState(null, document.title, new_url);
+	try {
+		// can fail when running from file: protocol
+		if (replace_history_state) {
+			history.replaceState(null, document.title, new_url);
+		} else {
+			history.pushState(null, document.title, new_url);
+		}
+	} catch(error) {
+		location.hash = new_hash;
 	}
 
 	$G.triggerHandler("change-url-params");

src/help.js

@@ -246,10 +246,31 @@ function open_help_viewer(options){
 		}
 	}
 
-	$.get(options.contentsFile, hhc => {
-		$($.parseHTML(hhc)).filter("ul").children().get().forEach((li)=> {
-			renderItem(li, null);
+	fetch(options.contentsFile).then((response)=> {
+		response.text().then((hhc)=> {
+			$($.parseHTML(hhc)).filter("ul").children().get().forEach((li)=> {
+				renderItem(li, null);
+			});
+		}, (error)=> {
+			show_error_message(`${localize("Failed to launch help.")} Failed to read ${options.contentsFile}.`, error);
 		});
+	}, (/* error */)=> {
+		// access to error message is not allowed either, basically
+		if (location.protocol === "file:") {
+			const $w = $FormToolWindow().title(localize("Paint")).addClass("dialogue-window");
+			$w.$main.html(`
+				<p>${localize("Failed to launch help.")}</p>
+				<p>This feature is not available when running from the <code>file:</code> protocol.</p>
+				<p>To use this feature, start a web server. If you have Python, you can use <code>python -m SimpleHTTPServer</code></p>
+			`);
+			$w.$main.css({maxWidth: "500px"});
+			$w.$Button(localize("OK"), () => {
+				$w.close();
+			});
+			$w.center();
+		} else {
+			show_error_message(`${localize("Failed to launch help.")} ${localize("Access to %1 was denied.", options.contentsFile)}`);
+		}
 	});
 	
 	// @TODO: keyboard accessability

src/tool-options.js

@@ -31,9 +31,10 @@ const ChooserCanvas = (
 			);
 		// eslint-disable-next-line no-empty
 		} catch(error) {}
-		if(invert){
-			invert_rgb(c.ctx);
-		}
+		// if(invert){
+		// 	invert_rgb(c.ctx); // can fail due to tainted canvas if running from file: protocol
+		// }
+		c.style.filter = invert ? "invert()" : "";
 	};
 	$(img).on("load", render);
 	render();