Make signature positioning native SWT for Wayland

[mkurz]

Summary

This removes the SWT/AWT embedding used by the visual signature positioning step and replaces the preview with a native SWT Canvas.

The change is intended to fix the Wayland-specific positioning failure tracked in:

• Fixes "Positioning" does not work under pure Wayland #170
• Fixes Linux: Missing Wayland support #66

Background

PDF-Over's main Linux UI already uses SWT/GTK, which can run as a native Wayland client. The positioning step was different: it created an SWT.EMBEDDED composite, called SWT_AWT.new_Frame(...), and embedded a Swing JPanel for the PDF preview/signature placement UI.

That mixed SWT/AWT path depends on X11/XEmbed-style embedding and does not work correctly when the surrounding SWT/GTK shell is running natively on Wayland. The visible result is that step 2 (Positionierung / Positioning) can stay grey and visual signature placement is not possible.

The same embedding is also related to the existing fallback path in PositioningState, which logs "Failed to create composite (probably a mac...)" and shows error.PositioningNotPossible ("Manual positioning currently not possible due to a Java Bug. Using automatic positioning."). This change removes the SWT_AWT dependency from positioning, so that specific failure mode should no longer apply. I have not tested macOS, so the fallback path is left in place.

Changes

Positioning Preview

• Rewrite SignaturePanel from Swing JPanel to SWT Canvas.
• Keep PDF rendering based on PDFBox PDFRenderer.
• Convert rendered BufferedImages to SWT ImageData using the existing ImageUtil.convertToSWT(...) helper.
• Pass SWT placeholder image data into the positioning preview instead of AWT image data.
• Remove the SWT.EMBEDDED / SWT_AWT.new_Frame(...) dependency from the positioning screen.
• Port mouse dragging, keyboard movement, page navigation, and mouse-wheel page switching to SWT events.
• Keep asynchronous PDF page rendering and discard stale render results via a generation counter.

Behaviour Changes Worth Reviewing

• Mouse-wheel page switching now accumulates the SWT wheel delta and switches pages once it reaches MOUSE_WHEEL_PAGE_THRESHOLD (3), which is one notch at the default "lines per scroll" setting. This replaces the old AWT workaround that de-duplicated wheel events by timestamp because they fired twice on Linux. Shift-wheel still jumps 5 pages.
• The sign button is now enabled once a signature position exists, rather than once the document has merely been handed to the viewer. This matters because the initial position is only known after the first page render completes.
• Render resolution is now derived from the monitor the shell is on (getShell().getMonitor()) instead of AWT's Toolkit.getScreenSize().

Lifecycle and Correctness Fixes

• Fix a crash when signing and exporting one PDF and then opening another: stale render and placeholder callbacks could touch already-disposed SWT widgets. These paths are now guarded, and PositioningComposite caches the Display instead of calling Widget.getDisplay() from worker-thread callbacks.
• PositioningState.closePDFDocument() now clears the viewer's document, synchronising with the render lock before closing the PDDocument, so a background render does not run against a closed document.
• Restoring a previously chosen position now waits until page geometry is known. Otherwise the restored coordinates could be clamped against a page size of zero and collapse to the page corner.
• Re-clamp the current signature position after page geometry changes, so the placeholder stays inside the page in PDFs whose pages have different sizes.
• Discard stale renders before doing PDFBox work rather than after, so paging quickly through a document no longer runs a full render for each skipped page.

Notes

The implementation still uses AWT classes for off-screen image processing and geometry where appropriate, such as BufferedImage and Point2D. It no longer creates or embeds AWT/Swing UI components for the positioning view.

Validation

Tested manually on Linux aarch64 in a native Wayland session using GDK_BACKEND=wayland:

• PDF rendered in step 2.
• Visual signature placement worked.
• Mouse-wheel page navigation worked.
• Signing and exporting a signed PDF worked.
• Opening another PDF after signing did not reproduce the disposed-widget crash seen during testing.
• Restoring a previous position worked.
• Switching between pages of different sizes kept the signature placeholder inside the page.

Not manually tested: X11, Windows, and macOS. The change uses portable SWT APIs and removes X11-specific SWT/AWT embedding from the positioning view, but I have not verified those platforms locally.

Also verified locally:

mvn -q -pl pdf-over-gui -am compile
mvn -q test
git diff --check upstream/main...HEAD