VSE: Keep zoom levels constant when resizing

We can apply the `V2D_KEEPZOOM` and `V2D_KEEPOFS_Y` flags to the main VSE
region to make sure this happens. If `V2D_KEEPZOOM` is set, we should update
the view2d's `oldwinx` and `oldwiny` in `ui_view2d_curRect_validate_resize`
just like the `V2D_KEEPASPECT` case.

Divide-by-zero is a real possibility here, since initialization status of
`oldwinx/y` cannot be determined from the `resize` boolean alone (e.g. loading
regions from disk on startup yields a `true` `resize` parameter, so any old
regions that have `KEEPZOOM` added through versioning will get this error since
their `oldwinx/y` is uninitialized). Avoid by initializing `oldwinx/y` if it is zero in
`ui_view2d_curRect_validate_resize`.

Addresses #136508.

Pull Request: https://projects.blender.org/blender/blender/pulls/137802

source/blender/blenkernel/BKE_blender_version.h

@@ -27,7 +27,7 @@
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 45
+#define BLENDER_FILE_SUBVERSION 46
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and cancel loading the file, showing a warning to

source/blender/blenloader/intern/versioning_400.cc

@@ -8834,6 +8834,25 @@ void blo_do_versions_400(FileData *fd, Library * /*lib*/, Main *bmain)
     FOREACH_NODETREE_END;
   }
 
+  if (!MAIN_VERSION_FILE_ATLEAST(bmain, 405, 46)) {
+    LISTBASE_FOREACH (bScreen *, screen, &bmain->screens) {
+      LISTBASE_FOREACH (ScrArea *, area, &screen->areabase) {
+        LISTBASE_FOREACH (SpaceLink *, sl, &area->spacedata) {
+          if (sl->spacetype == SPACE_SEQ) {
+            ListBase *regionbase = (sl == area->spacedata.first) ? &area->regionbase :
+                                                                   &sl->regionbase;
+            LISTBASE_FOREACH (ARegion *, region, regionbase) {
+              if (region->regiontype == RGN_TYPE_WINDOW) {
+                region->v2d.keepzoom |= V2D_KEEPZOOM;
+                region->v2d.keepofs |= V2D_KEEPOFS_X | V2D_KEEPOFS_Y;
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+
   /* Always run this versioning (keep at the bottom of the function). Meshes are written with the
    * legacy format which always needs to be converted to the new format on file load. To be moved
    * to a subversion check in 5.0. */

source/blender/editors/interface/view2d/view2d.cc

@@ -437,8 +437,14 @@ static void ui_view2d_curRect_validate_resize(View2D *v2d, bool resize)
   }
   winx = std::max<float>(winx, 1);
   winy = std::max<float>(winy, 1);
+  if (v2d->oldwinx == 0) {
+    v2d->oldwinx = winx;
+  }
+  if (v2d->oldwiny == 0) {
+    v2d->oldwiny = winy;
+  }
 
-  /* V2D_LIMITZOOM indicates that zoom level should be preserved when the window size changes */
+  /* V2D_KEEPZOOM indicates that zoom level should be preserved when the window size changes. */
   if (resize && (v2d->keepzoom & V2D_KEEPZOOM)) {
     float zoom, oldzoom;
 
@@ -581,12 +587,12 @@ static void ui_view2d_curRect_validate_resize(View2D *v2d, bool resize)
         height = width * winRatio;
       }
     }
-
-    /* store region size for next time */
-    v2d->oldwinx = short(winx);
-    v2d->oldwiny = short(winy);
   }
 
+  /* Store region size for next time. */
+  v2d->oldwinx = short(winx);
+  v2d->oldwiny = short(winy);
+
   /* Step 2: apply new sizes to cur rect,
    * but need to take into account alignment settings here... */
   if ((width != curwidth) || (height != curheight)) {

source/blender/editors/space_sequencer/space_sequencer.cc

@@ -194,8 +194,9 @@ static SpaceLink *sequencer_create(const ScrArea * /*area*/, const Scene *scene)
 
   region->v2d.scroll |= (V2D_SCROLL_BOTTOM | V2D_SCROLL_HORIZONTAL_HANDLES);
   region->v2d.scroll |= (V2D_SCROLL_RIGHT | V2D_SCROLL_VERTICAL_HANDLES);
-  region->v2d.keepzoom = 0;
-  region->v2d.keeptot = 0;
+  region->v2d.keepzoom = V2D_KEEPZOOM;
+  region->v2d.keepofs = V2D_KEEPOFS_X | V2D_KEEPOFS_Y;
+  region->v2d.keeptot = V2D_KEEPTOT_FREE;
   region->v2d.flag |= V2D_VIEWSYNC_AREA_VERTICAL;
   region->v2d.align = V2D_ALIGN_NO_NEG_Y;
 

source/blender/makesdna/DNA_view2d_types.h

@@ -47,7 +47,7 @@ typedef struct View2D {
   short winx, winy;
   /**
    * Storage of previous winx/winy values encountered by #UI_view2d_curRect_validate(),
-   * for keep-aspect.
+   * for V2D_KEEPZOOM and V2D_KEEPASPECT.
    */
   short oldwinx, oldwiny;