Include an optional screen size in the WindowsDesktop spec

[zmb3]

By default this should remain unspecified, which preserves the behavior we have today (Teleport uses the size of the user's browser window).

These new fields allow cluster administrators to configure desktops in such a way that they always use a particular screen size, ignoring the size passed from the browser client. This is necessary to accommodate some applications which require a very specific screen resolution in order to render correctly.

changelog: Desktops can now be configured to use the same screen resolution for all sessions.

[zmb3]

Not the prettiest - but seems to work as you'd expect:

[ibeckermayer]

The ultimate screen size is negotiated and then returned to us in handleRDPConnectionInitialized, so consider adding these changes there in order to alert the user in case what they requested was different from what was negotiated:

diff --git a/lib/srv/desktop/rdp/rdpclient/client.go b/lib/srv/desktop/rdp/rdpclient/client.go
index 38035c64c5..970761742e 100644
--- a/lib/srv/desktop/rdp/rdpclient/client.go
+++ b/lib/srv/desktop/rdp/rdpclient/client.go
@@ -72,6 +72,7 @@ import "C"
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"runtime/cgo"
 	"sync"
@@ -123,7 +124,12 @@ func init() {
 type Client struct {
 	cfg Config
 
-	// Parameters read from the TDP stream.
+	// `clientWidth` and `clientHeight` are initially set to either
+	// the values provided by the browser or the values provided by
+	// the config file (config file values take precedence). They are
+	// then used to request a screen size from the RDP server. Finally,
+	// they are set to the negotiated screen size returned from the RDP
+	// server, which is the actual screen size that will be used.
 	clientWidth, clientHeight uint16
 	username                  string
 
@@ -666,12 +672,36 @@ func cgo_handle_rdp_connection_initialized(
 func (c *Client) handleRDPConnectionInitialized(ioChannelID, userChannelID, screenWidth, screenHeight C.uint16_t) C.CGOErrCode {
 	c.cfg.Log.Debugf("Received RDP channel IDs: io_channel_id=%d, user_channel_id=%d", ioChannelID, userChannelID)
 	c.cfg.Log.Debugf("RDP server provided resolution of %dx%d", screenWidth, screenHeight)
+	negotiatedScreenWidth, negotiatedScreenHeight := uint16(screenWidth), uint16(screenHeight)
 
+	// If the server provided a different resolution than requested
+	if c.clientWidth != negotiatedScreenWidth || c.clientHeight != negotiatedScreenHeight {
+		changedResMsg := fmt.Sprintf("RDP negotiated a different resolution than requested: %dx%d -> %dx%d",
+			c.clientWidth, c.clientHeight, negotiatedScreenWidth, negotiatedScreenHeight)
+
+		// Update the client's bookkeeping variables.
+		c.clientWidth, c.clientHeight = negotiatedScreenWidth, negotiatedScreenHeight
+
+		// If the user requested a specific resolution via the config,
+		// warn the user about the resolution change.
+		if c.cfg.Width != 0 && c.cfg.Height != 0 {
+			c.cfg.Log.Warn(changedResMsg)
+			if err := c.cfg.Conn.WriteMessage(tdp.Notification{Message: changedResMsg, Severity: tdp.SeverityWarning}); err != nil {
+				c.cfg.Log.Errorf("failed handling RDPChannelIDs: %v", err)
+				return C.ErrCodeFailure
+			}
+		} else {
+			// Otherwise just log the resolution change at debug level.
+			c.cfg.Log.Debug(changedResMsg)
+		}
+	}
+
+	// Send the negotiated resolution and necessary channel ID's to the browser.
 	if err := c.cfg.Conn.WriteMessage(tdp.ConnectionInitialized{
 		IOChannelID:   uint16(ioChannelID),
 		UserChannelID: uint16(userChannelID),
-		ScreenWidth:   uint16(screenWidth),
-		ScreenHeight:  uint16(screenHeight),
+		ScreenWidth:   negotiatedScreenWidth,
+		ScreenHeight:  negotiatedScreenHeight,
 	}); err != nil {
 		c.cfg.Log.Errorf("failed handling RDPChannelIDs: %v", err)
 		return C.ErrCodeFailure

[public-teleport-github-review-bot]

@zmb3 See the table below for backport results.

Branch	Result
branch/v15	Failed