diff --git a/modules/desktop_capture/linux/screen_capturer_x11.cc b/modules/desktop_capture/linux/screen_capturer_x11.cc
index e05f7bfea7..c601a2afb8 100644
--- a/modules/desktop_capture/linux/screen_capturer_x11.cc
+++ b/modules/desktop_capture/linux/screen_capturer_x11.cc
@@ -325,6 +325,12 @@ bool ScreenCapturerX11::SelectSource(SourceId id) {
       selected_monitor_name_ = m.name;
       selected_monitor_rect_ =
           DesktopRect::MakeXYWH(m.x, m.y, m.width, m.height);
+      const auto& pixel_buffer_rect = x_server_pixel_buffer_.window_rect();
+      if (!pixel_buffer_rect.ContainsRect(selected_monitor_rect_)) {
+        RTC_LOG(LS_WARNING)
+            << "Cropping selected monitor rect to fit the pixel-buffer.";
+        selected_monitor_rect_.IntersectWith(pixel_buffer_rect);
+      }
       return true;
     }
   }
@@ -358,8 +364,10 @@ std::unique_ptr<DesktopFrame> ScreenCapturerX11::CaptureScreen() {
   RTC_DCHECK(selected_monitor_rect_.top_left().equals(frame->top_left()));
 
   // Pass the screen size to the helper, so it can clip the invalid region if it
-  // expands that region to a grid.
-  helper_.set_size_most_recent(x_server_pixel_buffer_.window_size());
+  // expands that region to a grid. Note that the helper operates in the
+  // DesktopFrame coordinate system where the top-left pixel is (0, 0), even for
+  // a monitor with non-zero offset relative to `x_server_pixel_buffer_`.
+  helper_.set_size_most_recent(frame->size());
 
   // In the DAMAGE case, ensure the frame is up-to-date with the previous frame
   // if any.  If there isn't a previous frame, that means a screen-resolution
@@ -382,15 +390,13 @@ std::unique_ptr<DesktopFrame> ScreenCapturerX11::CaptureScreen() {
       auto damage_rect = DesktopRect::MakeXYWH(rects[i].x, rects[i].y,
                                                rects[i].width, rects[i].height);
 
-      // Damage regions are in the same coordinate-system as
-      // ```selected_monitor_rect_```, but may fall outside of it.
-      damage_rect.IntersectWith(selected_monitor_rect_);
-      if (!damage_rect.is_empty()) {
-        // Convert to DesktopFrame coordinates where the top-left is
-        // always (0, 0), before adding to the frame's update_region.
-        damage_rect.Translate(-frame->top_left());
-        updated_region->AddRect(damage_rect);
-      }
+      // Damage-regions are relative to `x_server_pixel_buffer`, so convert the
+      // region to DesktopFrame coordinates where the top-left is always (0, 0),
+      // before adding to the frame's updated_region. `helper_` also operates in
+      // DesktopFrame coordinates, and it will take care of cropping away any
+      // damage-regions that lie outside the selected monitor.
+      damage_rect.Translate(-frame->top_left());
+      updated_region->AddRect(damage_rect);
     }
     XFree(rects);
     helper_.InvalidateRegion(*updated_region);