View2D: Inform region type on current view changes

Allows to hook per-space code which is to be run on view navigation.
This is required to have zoom-to-fit implemented in the sequencer.

There might be more cases where the clalback is to be called from,
but it could be easier to address those on the case-by-case basis
when its needed.

source/blender/blenkernel/BKE_screen.h

@@ -183,6 +183,16 @@ typedef struct ARegionType {
   /* return context data */
   int (*context)(const struct bContext *C, const char *member, struct bContextDataResult *result);
 
+  /* Is called whenever the current visible View2D's region changes.
+   *
+   * Used from user code such as view navigation/zoom operators to inform region about changes.
+   * The goal is to support zoom-to-fit features which gets disabled when manual navigation is
+   * performed.
+   *
+   * This callback is not called on indirect changes of the current viewport (which could happen
+   * when the `v2d->tot is changed and `cur` is adopted accordingly).  */
+  void (*on_view2d_changed)(const struct bContext *C, struct ARegion *region);
+
   /* custom drawing callbacks */
   ListBase drawcalls;
 

source/blender/editors/interface/view2d.c

@@ -853,9 +853,15 @@ void UI_view2d_curRect_validate(View2D *v2d)
   ui_view2d_curRect_validate_resize(v2d, false);
 }
 
-void UI_view2d_curRect_changed(const bContext *UNUSED(C), View2D *v2d)
+void UI_view2d_curRect_changed(const bContext *C, View2D *v2d)
 {
   UI_view2d_curRect_validate(v2d);
+
+  ARegion *region = CTX_wm_region(C);
+
+  if (region->type->on_view2d_changed != NULL) {
+    region->type->on_view2d_changed(C, region);
+  }
 }
 
 /* ------------------ */