During the window share, if the window is partially out of the screen, the capture won't be updated. If the window size is changed when the window is partially out of the screen, the capture will be black.

Since the window capture for windows that are partially out of the screen doesn't work. I suggest to remove them from the window list in preview dialog. 

@sergey

Since when a windows is partially out of screen, the window capture will stop. I plan to add a function to check whether the windows is partially out of screen when create the window list in preview. WDYT?

I think it's best to figure out why the capturer doesn't work when the target window is not on the screen and if we can fix this issue.
Currently the capturer calls XCopyArea() with src=window_. Maybe it's necessary to call XCompositeNameWindowPixmap() to get the backing pixmap and it would work better in case the window is not fully visible.

@sergeyu,

In normal/correct x11 window capture

https://cs.chromium.org/chromium/src/third_party/webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc?dr&q=XCopyArea+file:%5Esrc/third_party/webrtc/&sq=package:chromium&l=243

 if (shm_segment_info_) {
    if (shm_pixmap_) {
      XCopyArea(display_, window_, shm_pixmap_, shm_gc_,
                rect.left(), rect.top(), rect.width(), rect.height(),
                rect.left(), rect.top());
      XSync(display_, False);
    }
    data = reinterpret_cast<uint8_t*>(x_image_->data) +
        rect.top() * x_image_->bytes_per_line +
        rect.left() * x_image_->bits_per_pixel / 8;
  } else {

shm_segment_info_ is true but shm_pixmap_ is false and XCopyArea() is never called. Is this an issue?

MIT-SHM—The MIT Shared Memory Extension
https://www.x.org/archive/X11R7.5/doc/Xext/mit-shm.html
is used for window capture. 

XShmGetImage() will return false if the windows is partially out of screen.

Fixed has committed
https://codereview.chromium.org/2044693002/