RFR: 8354415: [Ubuntu25.04] api/java_awt/GraphicsDevice/indexTGF.html#SetDisplayMode - setDisplayMode_REFRESH_RATE_UNKNOWN fails: Height is different on vnc

[Sergey Bylokhov]

On Fri, 11 Jul 2025 15:47:33 GMT, Alexander Zvegintsev <azvegint at openjdk.org> wrote:

> On Linux, we are currently use XRRSetScreenConfigAndRate to change the display mode. This works in most cases, except for some VNC-configured hosts, where the display change may not occur.
> One of the conformance tests fails on these hosts.
> 
> It was discovered, that the xrandr utility app works fine on these hosts and uses the Crtc and Output Randr APIs, which provide more fine-grained control over display settings.
> 
> This changeset utilizes the Crtc and Output Randr APIs to change the display mode.
> 
> Currently, changing the display mode is only [available when running a single screen](https://github.com/openjdk/jdk/blob/a86dd56de34f730b42593236f17118ef5ce4985a/src/java.desktop/unix/classes/sun/awt/X11GraphicsDevice.java#L365).
> This limitation remains, but we may remove it in the future. A separate issue [has been filed](https://bugs.openjdk.org/browse/JDK-8361968) for this purpose.
> 
> The `awt.x11useOldConfigDisplayMode` system property enables the old behavior. Perhaps there's a better name for it.
> 
> Testing looks good on VNC and local machines.

src/java.desktop/unix/native/libawt_xawt/awt/awt_GraphicsEnv.c line 1986:

> 1984:         Bool matchH = (isLandscape ? mode.height : mode.width) == (unsigned int) height;
> 1985: 
> 1986:         if (matchW && matchH && rr == refreshRate) {

do we need to stop on the first mode we found or we should select the nearest refrate to the int? or maybe select the largest one? like 59.7 vs 59.9 vs 60?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26268#discussion_r2206543463