prevent jumps when image loads by adding a spacer

src/platform/web/ui/css/themes/element/theme.css

@@ -494,6 +494,8 @@ ul.Timeline > li.messageStatus .message-container > p {
 .message-container {
     padding: 1px 10px 0px 10px;
     margin: 5px 10px 0 10px;
+    /* so the .picture can grow horizontally and its spacer can grow vertically */
+    width: 100%;
 }
 
 .message-container .profile {
@@ -505,6 +507,10 @@ ul.Timeline > li.messageStatus .message-container > p {
     --avatar-size: 25px;
 }
 
+.TextMessageView {
+    width: 100%;
+}
+
 .TextMessageView.continuation .message-container {
     margin-top: 0;
     margin-bottom: 0;
@@ -534,22 +540,28 @@ ul.Timeline > li.messageStatus .message-container > p {
 }
 
 
-.message-container a.picture {
+.message-container .picture {
     display: grid;
     text-decoration: none;
     overflow: hidden;
     margin-top: 4px;
     border-radius: 4px;
+    width: 100%;
 }
 
-.message-container a.picture > img {
-    grid-row: 1 / 2;
-    grid-column: 1 / 2;
+/* .spacer grows with an inline padding-top to the size of the image,
+so the timeline doesn't jump when the image loads */
+.message-container .picture > * {
+    grid-row: 1;
+    grid-column: 1;
 }
 
-.message-container a.picture > time {
-    grid-row: 1 / 2;
-    grid-column: 1 / 2;
+.message-container .picture > img {
+    width: 100%;
+    height: auto;
+}
+
+.message-container .picture > time {
     align-self: end;
     justify-self: end;
     color: #2e2f32;
@@ -559,6 +571,9 @@ ul.Timeline > li.messageStatus .message-container > p {
     background-color: rgba(255, 255, 255, 0.75);
     border-radius: 4px;
 }
+.message-container .picture > .spacer {
+    width: 100%;
+}
 
 .TextMessageView.pending .message-container {
     color: #ccc;

src/platform/web/ui/css/timeline.css

@@ -37,14 +37,12 @@ limitations under the License.
     margin: 5px 0;
 }
 
-.message-container a.picture {
+.message-container .picture {
     display: block;
 }
 
-.message-container a.picture > img {
+.message-container .picture > img {
     display: block;
-    width: 100%;
-    height: auto;
 }
 
 .TextMessageView {

src/platform/web/ui/session/room/timeline/ImageView.js

@@ -19,20 +19,20 @@ import {renderMessage} from "./common.js";
 
 export class ImageView extends TemplateView {
     render(t, vm) {
-        return renderMessage(t, vm,
-            t.div([
-                t.a({href: vm.lightboxUrl, className: "picture"}, [
+        const heightRatioPercent = (vm.thumbnailHeight / vm.thumbnailWidth) * 100; 
+        return renderMessage(t, vm, [
+            t.a({href: vm.lightboxUrl, className: "picture", style: `max-width: ${vm.thumbnailWidth}px`}, [
+                t.div({className: "spacer", style: `padding-top: ${heightRatioPercent}%;`}),
                 t.img({
-                        src: vm => vm.thumbnailUrl,
                     loading: "lazy",
+                    src: vm => vm.thumbnailUrl,
                     alt: vm => vm.label,
                     title: vm => vm.label,
-                        style: vm => `max-width: ${vm.thumbnailWidth}px; max-height: ${vm.thumbnailHeight}px;`
+                    style: `max-width: ${vm.thumbnailWidth}px; max-height: ${vm.thumbnailHeight}px;`
                 }),
-                    t.time(vm.date + " " + vm.time)
+                t.time(vm.date + " " + vm.time),
             ]),
             t.if(vm => vm.error, t.createTemplate((t, vm) => t.p({className: "error"}, vm.error)))
-            ])
-        );
+        ]);
     }
 }