RFR: 8280861: Robot color picker broken on Linux with scaling above 100%

[Maxim Kartashev]

On Thu, 10 Feb 2022 18:04:37 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

>> The primary API for image capture on modern Linuxes is `gdk_pixbuf_get_from_window()` that expects both coordinates and the size unscaled (i.e. not multiplied by the current desktop scale). At the same time, `gtk3_interface.c:gtk3_get_drawable_data()` gets the coordinates from `Robot` that pre-scales them (multiplying by the scale known to Java, however, not necessarily the current desktop scale). The problem with the size had been partly taken care of in [JDK-8225118](https://bugs.openjdk.java.net/browse/JDK-8225118), but coordinates are still passed to `gdk_pixbuf_get_from_window()` pre-scaled.
>> 
>> The idea of the fix is to capture a possibly larger area that is guaranteed to contain the one that is interesting to the caller and then only copy the interesting pixels to the output image. As a positive side effect, the size of the captured area cannot be less than 1x1 (provided the correct input, of course). This solves the problem of zero size passed to  `(*fp_gdk_pixbuf_get_from_drawable)` when the desktop scale is 3 and we're asked to capture just one pixel. In that case, the previous formula (`width / (float) win_scale + 0.5`) would have yielded `0`. 
>> 
>> A related issue would be that tests written specifically for this general area (`java/awt/Robot`) didn't catch this problem and only somewhat unrelated tests (`javax/swing/...`) were affected.
>> 
>> This one is solved by adding pixel-sized areas to the test image in `HiDPIRobotScreenCaptureTest.java`, which precise colors are then verified to match the expected ones. In addition to that, instead of verifying just the color of the center of a large area, 4 more pixels close to the area's border are checked. This helps to make sure that captured area coordinates aren't way off.
>> 
>> The tests under `javax/swing/...` that originally helped to identify this problem were modified to provide more screen space for painting their components as extreme desktop scaling (300%) on Ubuntu doesn't leave much room due to a humongous title bar.
>> 
>> The fix was tested by running all the modified tests on Ubuntu with desktop scaling set to 100%, 200%, and 300%. The `Robot` tests were also executed on Windows (300% scaling) and MacOS.
>
> src/java.desktop/unix/native/libawt_xawt/awt/gtk3_interface.c line 2874:
> 
>> 2872: }
>> 2873: 
>> 2874: static gboolean gtk3_get_drawable_data(JNIEnv *env, jintArray pixelArray,
> 
> Don't we need to do the same for the gtk2? I suggest to update one of the test below to cover gtk_robot on/off for both gtk2/gtk3.

Maybe not as the previous fix was mode for gtk3 only, but I can double-check. Do you know if MATE desktop environment  is a good candidate for finding gtk2 or are there easier options on Ubuntu?

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PR: https://git.openjdk.java.net/jdk/pull/7425