webmail: improve dragging with mouse events over the message iframe

before, the iframe was consuming the mouse events, preventing the dragging to
the right from working properly. the workaround was to drag over the area with
the header, above the message iframe.

with this change, we disable pointer events over the entire right area, which
includes the iframe.

webmail/webmail.js

@@ -2133,17 +2133,23 @@ const refineFilters = (f, notf) => {
 // is the mousedown event. Move is the function to call when the bar was dragged,
 // typically adjusting styling, e.g. absolutely positioned offsets, possibly based
 // on the event.clientX and element bounds offset.
+// The returned promise is resolved when dragging is done (and immediately if
+// dragging wasn't activated).
 const startDrag = (e, move) => {
-	if (e.buttons === 1) {
+	if (e.buttons !== 1) {
+		return Promise.resolve();
+	}
+	return new Promise((resolve, _) => {
 		e.preventDefault();
 		e.stopPropagation();
 		const stop = () => {
 			document.body.removeEventListener('mousemove', move);
 			document.body.removeEventListener('mouseup', stop);
+			resolve();
 		};
 		document.body.addEventListener('mousemove', move);
 		document.body.addEventListener('mouseup', stop);
-	}
+	});
 };
 // Returns two handler functions: one for focus that sets a placeholder on the
 // target element, and one for blur that restores/clears it again. Keeps forms uncluttered,
@@ -6640,8 +6646,12 @@ const init = async () => {
 		}
 		if (want === 'leftright') {
 			let left, split, right;
-			dom._kids(splitElem, left = dom.div(css('layoutLeft', { position: 'absolute', left: 0, top: 0, bottom: 0 }), style({ width: 'calc(' + settings.leftWidthPct + '% - 1px)' }), msglistElem), split = dom.div(css('listMsgLeftRightGrab', { position: 'absolute', width: '5px', top: 0, bottom: 0, cursor: 'ew-resize', zIndex: zindexes.splitter }), style({ left: 'calc(' + settings.leftWidthPct + '% - 2px)' }), dom.div(css('listMsgLeftRightLine', { position: 'absolute', backgroundColor: styles.popupBorderColor, top: 0, bottom: 0, width: '1px', left: '2px', right: '2px' })), function mousedown(e) {
+			dom._kids(splitElem, left = dom.div(css('layoutLeft', { position: 'absolute', left: 0, top: 0, bottom: 0 }), style({ width: 'calc(' + settings.leftWidthPct + '% - 1px)' }), msglistElem), split = dom.div(css('listMsgLeftRightGrab', { position: 'absolute', width: '5px', top: 0, bottom: 0, cursor: 'ew-resize', zIndex: zindexes.splitter }), style({ left: 'calc(' + settings.leftWidthPct + '% - 2px)' }), dom.div(css('listMsgLeftRightLine', { position: 'absolute', backgroundColor: styles.popupBorderColor, top: 0, bottom: 0, width: '1px', left: '2px', right: '2px' })), async function mousedown(e) {
-				startDrag(e, (e) => {
+				// Disable pointer events on the message view. If it has an iframe with a message,
+				// mouse events while dragging would be consumed by the iframe, breaking our
+				// resize.
+				right.style.pointerEvents = 'none';
+				await startDrag(e, (e) => {
 					const bounds = left.getBoundingClientRect();
 					const x = Math.round(e.clientX - bounds.x);
 					left.style.width = 'calc(' + x + 'px - 1px)';
@@ -6650,6 +6660,7 @@ const init = async () => {
 					settingsPut({ ...settings, leftWidthPct: Math.round(100 * bounds.width / splitElem.getBoundingClientRect().width) });
 					updateMsglistWidths();
 				});
+				right.style.pointerEvents = '';
 			}), right = dom.div(css('layoutRight', { position: 'absolute', right: 0, top: 0, bottom: 0 }), style({ left: 'calc(' + settings.leftWidthPct + '% + 1px)' }), msgElem));
 		}
 		else {

webmail/webmail.ts

@@ -824,17 +824,23 @@ const refineFilters = (f: api.Filter, notf: api.NotFilter): [api.Filter, api.Not
 // is the mousedown event. Move is the function to call when the bar was dragged,
 // typically adjusting styling, e.g. absolutely positioned offsets, possibly based
 // on the event.clientX and element bounds offset.
-const startDrag = (e: MouseEvent, move: (e: MouseEvent) => void): void => {
+// The returned promise is resolved when dragging is done (and immediately if
-	if (e.buttons === 1) {
+// dragging wasn't activated).
+const startDrag = (e: MouseEvent, move: (e: MouseEvent) => void): Promise<void> => {
+	if (e.buttons !== 1) {
+		return Promise.resolve()
+	}
+	return new Promise((resolve, _) => {
 		e.preventDefault()
 		e.stopPropagation()
 		const stop = () => {
 			document.body.removeEventListener('mousemove', move)
 			document.body.removeEventListener('mouseup', stop)
+			resolve()
 		}
 		document.body.addEventListener('mousemove', move)
 		document.body.addEventListener('mouseup', stop)
-	}
+	})
 }
 
 // Returns two handler functions: one for focus that sets a placeholder on the
@@ -6880,8 +6886,12 @@ const init = async () => {
 					css('listMsgLeftRightGrab', {position: 'absolute', width: '5px', top: 0, bottom: 0, cursor: 'ew-resize', zIndex: zindexes.splitter}),
 					style({left: 'calc(' + settings.leftWidthPct + '% - 2px)'}),
 					dom.div(css('listMsgLeftRightLine', {position: 'absolute', backgroundColor: styles.popupBorderColor, top: 0, bottom: 0, width: '1px', left: '2px', right: '2px'})),
-					function mousedown(e: MouseEvent) {
+					async function mousedown(e: MouseEvent) {
-						startDrag(e, (e) => {
+						// Disable pointer events on the message view. If it has an iframe with a message,
+						// mouse events while dragging would be consumed by the iframe, breaking our
+						// resize.
+						right.style.pointerEvents = 'none'
+						await startDrag(e, (e) => {
 							const bounds = left.getBoundingClientRect()
 							const x = Math.round(e.clientX - bounds.x)
 							left.style.width = 'calc(' + x +'px - 1px)'
@@ -6890,6 +6900,7 @@ const init = async () => {
 							settingsPut({...settings, leftWidthPct: Math.round(100*bounds.width/splitElem.getBoundingClientRect().width)})
 							updateMsglistWidths()
 						})
+						right.style.pointerEvents = ''
 					}
 				),
 				right=dom.div(