How to Prevent Window from Gaining Input Focus in X11

• The override-redirect flag tells the window manager to ignore any requests from the window to change its position, size, stacking order, or input focus.
• The article shows how to set and unset this flag on an existing window using XCB, a low-level library for interacting with X11.
• The article also discusses some of the implications and limitations of using this flag, and answers some frequently asked questions related to this topic.

If you are developing an application that uses X11 to communicate with the X server, you may encounter a situation where you want to send “fake” mouse or keyboard events to a window, but you don’t want the window to gain input focus. For example, you may want to control a window that displays some graphics or animations, while still being able to interact with other windows on your screen.

However, by default, whenever you send a mouse click or a keyboard press to a window, the window manager will give it the input focus, which means that it will receive all the subsequent keyboard and mouse events. This can be annoying, as it may interfere with your workflow and cause the window to be raised above other windows.

Fortunately, there is a way to prevent this behavior by using the override-redirect flag. This flag tells the window manager to ignore any requests from the window to change its position, size, stacking order, or input focus. This way, you can send “fake” events to the window without affecting its focus state.

In this article, we will show you how to set and unset the override-redirect flag on an existing window using XCB, a low-level library for interacting with X11. We will also explain some of the implications and limitations of using this flag.

Setting the Override-Redirect Flag

To set the override-redirect flag on an existing window, you need to use the xcb_change_window_attributes function. This function allows you to change various attributes of a window, such as its background color, event mask, or border width. One of these attributes is the override-redirect flag, which is represented by the constant XCB_CW_OVERRIDE_REDIRECT.

The xcb_change_window_attributes function takes four parameters:

• A pointer to an xcb_connection_t structure, which represents the connection to the X server.
• The ID of the window whose attributes you want to change.
• A bitmask that specifies which attributes you want to change. You can use bitwise OR operations to combine multiple attributes.
• A pointer to an array of values that correspond to the attributes you want to change.

To set the override-redirect flag on a window, you need to pass XCB_CW_OVERRIDE_REDIRECT as the bitmask parameter, and 1 as the value parameter. For example:

// Get the connection to the X server
xcb_connection_t *conn = xcb_connect(NULL, NULL);

// Get the ID of the window you want to control
xcb_window_t win = ...;

// Set the override-redirect flag
uint32_t value = 1;
xcb_change_window_attributes(conn, win, XCB_CW_OVERRIDE_REDIRECT, &value);

// Flush the request to the X server
xcb_flush(conn);

This code will make the window ignore any requests from the window manager regarding its position, size, stacking order, or input focus. You can then send “fake” mouse or keyboard events to it using functions like xcb_test_fake_input or xcb_send_event.

Unsetting the Override-Redirect Flag

To unset the override-redirect flag on a window, you need to use the same function as before, but pass 0 as the value parameter. For example:

// Get the connection to the X server
xcb_connection_t *conn = xcb_connect(NULL, NULL);

// Get the ID of the window you want to control
xcb_window_t win = ...;

// Unset the override-redirect flag
uint32_t value = 0;
xcb_change_window_attributes(conn, win, XCB_CW_OVERRIDE_REDIRECT, &value);

// Flush the request to the X server
xcb_flush(conn);

This code will restore the normal behavior of the window manager regarding the window’s position, size, stacking order, and input focus.

Implications and Limitations

Using the override-redirect flag can be useful for some applications that need to control windows without affecting their focus state. However, there are some implications and limitations that you should be aware of before using it.

First of all, setting this flag means that you are bypassing some of the features and policies of your window manager. This may cause some unexpected or undesired results depending on your window manager’s configuration and behavior. For example:

• The window may not be decorated with a title bar or a border by your window manager.
• The window may not be visible in your taskbar or pager by your window manager.
• The window may not respond to some keyboard shortcuts or mouse actions defined by your window manager.
• The window may not follow some rules or hints set by your window manager or other applications regarding its position, size, stacking order, or input focus.

Secondly, unsetting this flag does not necessarily mean that your window will regain its previous focus state. Depending on your window manager’s focus policy and history, your window may not receive input focus automatically after unsetting this flag. You may need to explicitly request input focus for your window using functions like xcb_set_input_focus or xcb_send_event.

Thirdly, setting this flag does not guarantee that your window will always receive the “fake” events that you send to it. Depending on your window manager’s configuration and behavior, your window may still be occluded or obscured by other windows, which may prevent it from receiving some of the events. You may need to use functions like xcb_configure_window or xcb_circulate_window to change the position or stacking order of your window to make it visible and accessible.

Frequently Asked Questions

Here are some common questions and answers related to the topic of this article.

Question: How can I get the ID of the window I want to control?

Answer: There are different ways to get the ID of a window in X11. One way is to use a tool like xwininfo or xprop, which can display various information about a window, including its ID. You can run these tools from a terminal and click on the window you want to inspect. Another way is to use a function like xcb_query_tree, which can return a list of all the windows that are children of a given window. You can use this function to traverse the window hierarchy and find the window you want by its name, class, or other attributes.

Question: How can I send “fake” mouse or keyboard events to a window?

Answer: There are different ways to send “fake” mouse or keyboard events to a window in X11. One way is to use a function like xcb_test_fake_input, which can generate various types of events, such as button presses, button releases, key presses, key releases, motion events, etc. You can use this function to simulate user input for a window. Another way is to use a function like xcb_send_event, which can send any type of event to a window. You can use this function to send events that are not normally generated by user input, such as focus events, expose events, configure events, etc.

Question: How can I request input focus for a window?

Answer: There are different ways to request input focus for a window in X11. One way is to use a function like xcb_set_input_focus, which can set the input focus to a given window. You can use this function to explicitly request input focus for your window after unsetting the override-redirect flag. Another way is to use a function like xcb_send_event, which can send any type of event to a window. You can use this function to send a synthetic event that mimics a user action that would normally give input focus to your window, such as clicking on it or pressing Alt-Tab.

Conclusion

In this article, we have shown you how to prevent a window from gaining input focus in X11 by using the override-redirect flag. We have explained how to set and unset this flag on an existing window using XCB, and what are some of the implications and limitations of using it. We have also answered some frequently asked questions related to this topic.

We hope you have found this article useful and informative. If you have any questions or feedback, please feel free to leave a comment below. Thank you for reading!

Disclaimer: The content of this article is for educational and informational purposes only. It is not intended to provide professional advice or solutions for any specific problem or situation. The author and the publisher are not responsible for any errors, omissions, or consequences that may arise from the use of the information or code presented in this article. The user should always consult with a qualified expert before applying any of the techniques or methods described in this article. The user should also verify the accuracy and validity of the information and code provided in this article before using them. The user assumes all risks and liabilities for their actions and decisions based on the content of this article.