UI: Refactor how draggable part of button is determined

No longer use the existance of an image pointer inside the button, or
the the type of a button to decide if the entire button should be
draggable, or only its icon. This was rather sneaky, undocumented
behavior. Instead make this a proper option in the public UI API, and
enable it by default in specific cases to match the previous behavior.

This at least makes the behavior known and controllable in the API, and
not something that's just "secretly" done deep down in the button
handling code. I'd argue we should just use the entire button width by
default for dragging, but that would be a bigger change.

source/blender/editors/include/UI_interface.h

@@ -239,6 +239,17 @@ enum {
   UI_BUT_OVERRIDDEN = 1u << 31u,
 };
 
+/** #uiBut.dragflag */
+enum {
+  /** By default only the left part of a button triggers dragging. A questionable design to make
+   * the icon but not other parts of the button draggable. Set this flag so the entire button can
+   * be dragged. */
+  UI_BUT_DRAG_FULL_BUT = (1 << 0),
+
+  /* --- Internal flags. --- */
+  UI_BUT_DRAGPOIN_FREE = (1 << 1),
+};
+
 /* Default font size for normal text. */
 #define UI_DEFAULT_TEXT_POINTS 11.0f
 
@@ -881,6 +892,9 @@ bool UI_but_flag_is_set(uiBut *but, int flag);
 void UI_but_drawflag_enable(uiBut *but, int flag);
 void UI_but_drawflag_disable(uiBut *but, int flag);
 
+void UI_but_dragflag_enable(uiBut *but, int flag);
+void UI_but_dragflag_disable(uiBut *but, int flag);
+
 void UI_but_disable(uiBut *but, const char *disabled_hint);
 
 void UI_but_type_set_menu_from_pulldown(uiBut *but);
@@ -1789,9 +1803,12 @@ void UI_but_drag_set_id(uiBut *but, struct ID *id);
 /**
  * Set an image to display while dragging. This works for any drag type (`WM_DRAG_XXX`).
  * Not to be confused with #UI_but_drag_set_image(), which sets up dragging of an image.
+ *
+ * Sets #UI_BUT_DRAG_FULL_BUT so the full button can be dragged.
  */
 void UI_but_drag_attach_image(uiBut *but, struct ImBuf *imb, float scale);
 /**
+ * Sets #UI_BUT_DRAG_FULL_BUT so the full button can be dragged.
  * \param asset: May be passed from a temporary variable, drag data only stores a copy of this.
  */
 void UI_but_drag_set_asset(uiBut *but,
@@ -1808,6 +1825,8 @@ void UI_but_drag_set_name(uiBut *but, const char *name);
  * Value from button itself.
  */
 void UI_but_drag_set_value(uiBut *but);
+
+/** Sets #UI_BUT_DRAG_FULL_BUT so the full button can be dragged. */
 void UI_but_drag_set_image(
     uiBut *but, const char *path, int icon, struct ImBuf *imb, float scale, bool use_free);
 

source/blender/editors/interface/interface.cc

@@ -4224,6 +4224,10 @@ static uiBut *ui_def_but(uiBlock *block,
     but->flag |= UI_BUT_UNDO;
   }
 
+  if (ELEM(but->type, UI_BTYPE_COLOR)) {
+    but->dragflag |= UI_BUT_DRAG_FULL_BUT;
+  }
+
   BLI_addtail(&block->buttons, but);
 
   if (block->curlayout) {
@@ -5890,6 +5894,16 @@ void UI_but_drawflag_disable(uiBut *but, int flag)
   but->drawflag &= ~flag;
 }
 
+void UI_but_dragflag_enable(uiBut *but, int flag)
+{
+  but->dragflag |= flag;
+}
+
+void UI_but_dragflag_disable(uiBut *but, int flag)
+{
+  but->dragflag &= ~flag;
+}
+
 void UI_but_disable(uiBut *but, const char *disabled_hint)
 {
   UI_but_flag_enable(but, UI_BUT_DISABLED);

source/blender/editors/interface/interface_drag.cc

@@ -24,6 +24,7 @@ void UI_but_drag_attach_image(uiBut *but, struct ImBuf *imb, const float scale)
 {
   but->imb = imb;
   but->imb_scale = scale;
+  UI_but_dragflag_enable(but, UI_BUT_DRAG_FULL_BUT);
 }
 
 void UI_but_drag_set_asset(uiBut *but,

source/blender/editors/interface/interface_intern.hh

@@ -86,11 +86,6 @@ enum {
   /* WARNING: rest of #uiBut.flag in UI_interface.h */
 };
 
-/** #uiBut.dragflag */
-enum {
-  UI_BUT_DRAGPOIN_FREE = (1 << 0),
-};
-
 /** #uiBut.pie_dir */
 enum RadialDirection {
   UI_RADIAL_NONE = -1,

source/blender/editors/interface/interface_query.cc

@@ -247,7 +247,7 @@ bool ui_but_contains_point_px_icon(const uiBut *but, ARegion *region, const wmEv
 
   BLI_rcti_rctf_copy(&rect, &but->rect);
 
-  if (but->imb || but->type == UI_BTYPE_COLOR) {
+  if (but->dragflag & UI_BUT_DRAG_FULL_BUT) {
     /* use button size itself */
   }
   else if (but->drawflag & UI_BUT_ICON_LEFT) {