Fix running into m.redraw.sync() lock in MailViewer

Stream.map() callback would be called immediately during setViewModel()
because of how streams are implemented, and we would try to do
m.redraw.sync() inside of another redraw.

Now we schedule a microtask after the current redraw.

fix #4570

src/mail/view/MailViewer.ts

@@ -181,6 +181,10 @@ export class MailViewer implements Component<MailViewerAttrs> {
 		if (this.viewModel !== oldViewModel) {
 			this.loadAllListener.end(true)
 			this.loadAllListener = this.viewModel.loadCompleteNotification.map(async () => {
+				// streams are pretty much synchronous, so we could be in the middle of a redraw here and mithril does not just schedule another redraw, it
+				// will error out so before calling m.redraw.sync() we want to make sure that we are not inside a redraw by just scheduling a microtask with
+				// this simple await.
+				await Promise.resolve()
 				// Wait for mail body to be redrawn before replacing images
 				m.redraw.sync()
 				await this.replaceInlineImages()
@@ -451,14 +455,14 @@ export class MailViewer implements Component<MailViewerAttrs> {
 
 	private renderLoadingIcon(): Children {
 		return m(".progress-panel.flex-v-center.items-center",
-				{
-					key: "loadingIcon",
-					style: {
-						height: "200px",
-					},
+			{
+				key: "loadingIcon",
+				style: {
+					height: "200px",
 				},
-				[progressIcon(), m("small", lang.get("loading_msg"))],
-			)
+			},
+			[progressIcon(), m("small", lang.get("loading_msg"))],
+		)
 	}
 
 	private renderBanners(mail: Mail): Children {
@@ -1002,7 +1006,7 @@ export class MailViewer implements Component<MailViewerAttrs> {
 							),
 							m(".flex",
 								m(ExpanderButton, {
-									style: { paddingTop: "0px" },
+									style: {paddingTop: "0px"},
 									label: "showAll_action",
 									expanded: this.filesExpanded(),
 									onExpandedChange: this.filesExpanded,