Ignore resize observation caused by server resizes

If we increase the remote screen size from the server in such a way that
it no longer fits the browser window, the browser will probably want to
show scrollbars. The same happens if you enable 'clipping' while the
remote is larger than the browser window. These scrollbars do, in turn,
decrease the available space in the browser window. This causes our
ResizeObserver to trigger.

If the resize observation triggers a requestRemoteResize() we will
overwrite the size and request a new one just because scrollbars have
appeared. We don't want that.

We can save the expected client size after resizing, and then compare
the current client size with the expected one. If there is no change
compared to the expected size, we shouldn't send the request.

Fixes issue #1616.

core/rfb.js

@@ -244,6 +244,8 @@ export default class RFB extends EventTargetMixin {
         this._sock.on('message', this._handleMessage.bind(this));
         this._sock.on('error', this._socketError.bind(this));
 
+        this._expectedClientWidth = null;
+        this._expectedClientHeight = null;
         this._resizeObserver = new ResizeObserver(this._eventHandlers.handleResize);
 
         // All prepared, kick off the connection
@@ -623,7 +625,26 @@ export default class RFB extends EventTargetMixin {
             { detail: { name: this._fbName } }));
     }
 
+    _saveExpectedClientSize() {
+        this._expectedClientWidth = this._screen.clientWidth;
+        this._expectedClientHeight = this._screen.clientHeight;
+    }
+
+    _currentClientSize() {
+        return [this._screen.clientWidth, this._screen.clientHeight];
+    }
+
+    _clientHasExpectedSize() {
+        const [currentWidth, currentHeight] = this._currentClientSize();
+        return currentWidth == this._expectedClientWidth &&
+            currentHeight == this._expectedClientHeight;
+    }
+
     _handleResize() {
+        // Don't change anything if the client size is already as expected
+        if (this._clientHasExpectedSize()) {
+            return;
+        }
         // If the window resized then our screen element might have
         // as well. Update the viewport dimensions.
         window.requestAnimationFrame(() => {
@@ -664,6 +685,12 @@ export default class RFB extends EventTargetMixin {
             this._display.viewportChangeSize(size.w, size.h);
             this._fixScrollbars();
         }
+
+        // When changing clipping we might show or hide scrollbars.
+        // This causes the expected client dimensions to change.
+        if (curClip !== newClip) {
+            this._saveExpectedClientSize();
+        }
     }
 
     _updateScale() {
@@ -688,6 +715,7 @@ export default class RFB extends EventTargetMixin {
         }
 
         const size = this._screenSize();
+
         RFB.messages.setDesktopSize(this._sock,
                                     Math.floor(size.w), Math.floor(size.h),
                                     this._screenID, this._screenFlags);
@@ -2507,6 +2535,9 @@ export default class RFB extends EventTargetMixin {
         this._updateScale();
 
         this._updateContinuousUpdates();
+
+        // Keep this size until browser client size changes
+        this._saveExpectedClientSize();
     }
 
     _xvpOp(ver, op) {