Fixes crash on pinned window followed by darkroom navigation

[masterpiga]

Summary

Fixes a crash that occurs when pinning an image in the second darkroom window and navigating images in the main view, and hardens viewport dimension calculations against division-by-zero during image transitions.

Problem & Root Cause

1. GtkGesture Signal Signature Mismatch:
   In GTK 3, the GtkGesture::begin and GtkGesture::end signals pass three arguments:
   (GtkGesture *gesture, GdkEventSequence *sequence, gpointer user_data).
   The callback handlers omitted the sequence parameter ((GtkGestureZoom *gesture, gpointer user_data)). For touchpad gestures, GTK emits a NULL sequence; on ARM64, this placed 0x0 into the second argument register (x1), causing the callback to receive dev = NULL. Accessing dev->preview2_pinned (at offset 0xba0 in dt_develop_t) caused an immediate null pointer dereference.
2. Intermediate Loading State Divisor Checks:
   During image switches while second window callbacks are active, dt_dev_get_processed_size could return 0 for width/height before pipeline nodes are ready. Calling functions (dt_dev_get_zoom_scale, _dev_zoom_move, dt_dev_get_pointer_zoom_pos) did not guard against procw <= 0 or proch <= 0, leading to potential division by zero / inf / NaN.
3. Viewport Fallback:
   dt_dev_get_processed_size always fell back to darktable.develop->preview_pipe instead of checking port->dev->preview_pipe when dealing with a pinned develop instance.

Changes

• src/gui/gtk.c:
  • Corrected _pinch_begin and _pinch_end callback signatures to include GdkEventSequence *sequence.
• src/views/darkroom.c:
  • Added !dev checks across all second-window event callbacks (_second_window_pinch, button pressed/released, pan, mouse moved, touch moved, leave).
• src/develop/develop.c:
  • Updated dt_dev_get_processed_size fallback to respect port->dev when available.
  • Guarded against procw <= 0, proch <= 0, and scale <= 0.0f in dt_dev_get_zoom_scale, _dev_zoom_move, and dt_dev_get_pointer_zoom_pos / dt_dev_get_pointer_zoom_pos_from_bounds.

Verification seqeuence

• Open second darkroom window and toggle the pin button on an image.
• Advance images in the main darkroom view (e.g. with Space / Backspace).
• Zoom and pan in both the main view and the pinned second window using touchpad pinch, wheel scroll, and drag.
• Unpin and verify normal second window synchronization.

Crash report

crash_report.txt

Co-authored with Gemini.

[TurboGit]

Thanks!