Fix incorrect cursor scaling by using autoscale zoom

[ShahzaibIbrahim]

---

🐞 Problem Description

In SWT, custom cursors were being created with incorrect sizes on high-DPI displays. The root cause was this line:

OS.SetCursor(Cursor.win32_getHandle(cursor, getNativeZoom()));

The method Cursor.win32_getHandle(...) used getNativeZoom() to determine the scale at which the cursor image should be rendered.

However, getNativeZoom() only reflects the raw device DPI scaling (e.g., 125%, 150%), and does not account for SWT’s autoscale policy (controlled via the swt.autoScale property).

This leads to inconsistencies:

• Shells, fonts, and other UI elements in SWT may scale according to a rounded zoom factor (e.g., 100%, 200%),
• But the cursor was scaled using the unrounded native zoom, causing it to appear too small or too large compared to the rest of the UI.

---

✅ The Fix

To align cursor scaling with the rest of the SWT UI, the cursor zoom should be computed using the same logic that SWT uses for autoscaling across the toolkit.

Replace:

Cursor.win32_getHandle(cursor, getNativeZoom())

With:

Cursor.win32_getHandle(cursor, DPIUtil.getZoomForAutoscaleProperty(getNativeZoom()))

This uses:

• DPIUtil.getZoomForAutoscaleProperty(...), which applies the logic from swt.autoScale (e.g., rounding zoom levels up to the nearest 100%)
• Ensures cursor size matches the size expectations of other UI components (like Shells, Fonts, Images)

---

Various Cases with Snippets

public class Snippet {
	public static void main(String[] args) {
		//System.setProperty("swt.autoScale", "int200");
		Display display = Display.getDefault();
		Cursor cursor = new Cursor(display, getImageData(100), 0, 0);
		Shell shell = new Shell(display, SWT.NO_TRIM);
		shell.setSize(100, 100);
		shell.setCursor(cursor);
		shell.setBackground(new Color(0, 255, 0));
		shell.open();
		while (!display.isDisposed()) {
			display.readAndDispatch();
		}
		cursor.dispose();
	}

	private static ImageData getImageData(int zoom) {
		Image image = new Image(null, 50, 50);
		try {
			GC gc = new GC(image);
			gc.setBackground(new Color(255, 0, 0));
			gc.fillRectangle(0, 0, 50, 50);
			return image.getImageData(zoom);
		} finally {
			image.dispose();
		}
	}
}

- autoScale=1000

- autoScale=quarter, zoom = 175%

- autoScale=int200, zoom = 175%

[github-actions]

Test Results

   546 files  + 7     546 suites  +7   30m 8s ⏱️ - 1m 40s
 4 407 tests +49   4 390 ✅ +47   17 💤 +3  0 ❌  - 1 
16 713 runs  +49  16 586 ✅ +47  127 💤 +3  0 ❌  - 1 

Results for commit cebe524. ± Comparison against base commit 2853d53.

♻️ This comment has been updated with latest results.

[amartya4256]

I have tested it for 175, 100 and 125% on both master and this branch.
My finding:

1. On master the cursor takes the size of the native zoom.
2. With this PR, the cursor is consistent with the zoom (device zoom/autoscaledZoom/specific to property, e.g.: quarter, half, etc)

Hence, approving it.

[fedejeanne]

The code looks fine, I'm just confused about which issue does this PR solve (and how to test it). Is it #2308 ?
The screenshots you provided look similar to the ones in the issue but I don't see the "cursor" in them (the red square).

[fedejeanne]

The code looks fine, I'm just confused about which issue does this PR solve (and how to test it). Is it #2308 ?

I see that this is documented in our internal issue (vi-eclipse/Eclipse-Platform#357). You just need to adapt the text of the commit and this PR's description to say that if fixes #2308.

The screenshots you provided look similar to the ones in the issue but I don't see the "cursor" in them (the red square).

Scratch that, I see now that they are GIFs and they ran only once so I missed the red square. It's there though 👍

[fedejeanne]

The API check failure is unrelated.