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Usually, when I plug in a second monitor and I want my computer to be aware of it, I have to go to 'System' -> 'Preferences' -> 'Monitors'. Then, without clicking anywhere, my desktop knows it has to adapt itself (by cloning the screen or broadening the virtual desktop).

How can I make this happen automatically ?

For more information, my computer is an Acer Extensa 5636 notebook plugged to a Compaq CQ1859s monitor, using a VGA cable.

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you probably 'could' make this automatic but I'm not sure you would want to. Detecting changes can cause the screens to flicker and I don't think thats something you want happening automatically. –  WalterJ89 Feb 20 '11 at 12:50
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AFAIK, you cant. And thats a hardware restriction, not Ubuntu's.

It has to do with the way videocards (onboard or not) deal with its connectors: when they are unpplugged, the card "shuts down" the connector.

When you open Monitors, it "scans" the card(s) for the plugged monitors, thus "awakening" the connectors.

VGA (or DVI) ports are not like USB... theres no "pooling" to check for newly plugged devices, and theres no "IRQ" for the monitors to signal themselves. Its not plug-and-play. Its all up to the video card itself. And they usually dont do pooling.

A few years back it was even worse: many cards required the monitors to be plugged in on power up. So I had to reboot my PC for it to "find" the plugged monitor.

A wild shot here: have you tried HDMI connector? Since it carries sound, theres a chance your video card handles it in a more "flexible" way, giving the OS more control over plugged monitors.

If you want more detailed tech info, give me your card and motherboard model, monitors used, and, more important, conectors you're using for each.

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It bears noting that MacOS contains the functionality of automatically addressing plugged-in monitors. I'd imagine that this would therefore be plausible to achieve through the OS. –  user6658 Mar 13 '11 at 6:04
    
It is achievable by the OS, using the same method that 'System' -> 'Preferences' -> 'Monitors' uses. The problem lies in doing it automatically when a monitor is plugged in, since there is no generic way for connector to signal the OS for such event. Maybe MacOS has some sort of daemon constantly pools the connectors, OR the (proprietaty) video driver itself handles this signaling. –  MestreLion Mar 16 '11 at 5:19
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