Cleanup: Early-exit in button handling code, minor cleanups

This should decrease cognitive load because:
- Intention is more explicit
- Shorter visual scope of branches (no need to search for matching
  closing brackets and `else` blocks)
- Visually less busy code because condition-checks and code that
  "actually does things" are separated, with less indentation
- Avoids chaining together a bunch of conditions to a single `if`

Also: Use `const`, correct comment placement, whitespace improvements.

source/blender/editors/interface/interface_handlers.c

@@ -542,7 +542,7 @@ static bool but_copypaste_profile_alive = false;
 
 bool ui_but_is_editing(const uiBut *but)
 {
-  uiHandleButtonData *data = but->active;
+  const uiHandleButtonData *data = but->active;
   return (data && ELEM(data->state, BUTTON_STATE_TEXT_EDITING, BUTTON_STATE_NUM_EDITING));
 }
 
@@ -660,21 +660,23 @@ static bool ui_but_dragedit_update_mval(uiHandleButtonData *data, int mx)
 static bool ui_rna_is_userdef(PointerRNA *ptr, PropertyRNA *prop)
 {
   /* Not very elegant, but ensures preference changes force re-save. */
-  bool tag = false;
+
-  if (prop && !(RNA_property_flag(prop) & PROP_NO_DEG_UPDATE)) {
+  if (!prop) {
+    return false;
+  }
+  if (RNA_property_flag(prop) & PROP_NO_DEG_UPDATE) {
+    return false;
+  }
+
   StructRNA *base = RNA_struct_base(ptr->type);
   if (base == NULL) {
     base = ptr->type;
   }
-    if (ELEM(base,
+  return ELEM(base,
               &RNA_AddonPreferences,
               &RNA_KeyConfigPreferences,
               &RNA_KeyMapItem,
-             &RNA_UserAssetLibrary)) {
+              &RNA_UserAssetLibrary);
-      tag = true;
-    }
-  }
-  return tag;
 }
 
 bool UI_but_is_userdef(const uiBut *but)
@@ -900,7 +902,10 @@ static void ui_apply_but_func(bContext *C, uiBut *but)
 /* typically call ui_apply_but_undo(), ui_apply_but_autokey() */
 static void ui_apply_but_undo(uiBut *but)
 {
-  if (but->flag & UI_BUT_UNDO) {
+  if (!(but->flag & UI_BUT_UNDO)) {
+    return;
+  }
+
   const char *str = NULL;
   size_t str_len_clip = SIZE_MAX - 1;
   bool skip_undo = false;
@@ -958,7 +963,6 @@ static void ui_apply_but_undo(uiBut *but)
   uiAfterFunc *after = ui_afterfunc_new();
   BLI_strncpy(after->undostr, str, min_zz(str_len_clip + 1, sizeof(after->undostr)));
 }
-}
 
 static void ui_apply_but_autokey(bContext *C, uiBut *but)
 {
@@ -967,15 +971,16 @@ static void ui_apply_but_autokey(bContext *C, uiBut *but)
   /* try autokey */
   ui_but_anim_autokey(C, but, scene, scene->r.cfra);
 
-  /* make a little report about what we've done! */
+  if (!but->rnaprop) {
-  if (but->rnaprop) {
+    return;
-    char *buf;
+  }
 
   if (RNA_property_subtype(but->rnaprop) == PROP_PASSWORD) {
     return;
   }
 
-    buf = WM_prop_pystring_assign(C, &but->rnapoin, but->rnaprop, but->rnaindex);
+  /* make a little report about what we've done! */
+  char *buf = WM_prop_pystring_assign(C, &but->rnapoin, but->rnaprop, but->rnaindex);
   if (buf) {
     BKE_report(CTX_wm_reports(C), RPT_PROPERTY, buf);
     MEM_freeN(buf);
@@ -983,7 +988,6 @@ static void ui_apply_but_autokey(bContext *C, uiBut *but)
     WM_event_add_notifier(C, NC_SPACE | ND_SPACE_INFO_REPORT, NULL);
   }
 }
-}
 
 static void ui_apply_but_funcs_after(bContext *C)
 {
@@ -1631,14 +1635,23 @@ static bool ui_drag_toggle_set_xy_xy(
     LISTBASE_FOREACH (uiBut *, but, &block->buttons) {
       /* NOTE: ctrl is always true here because (at least for now)
        * we always want to consider text control in this case, even when not embossed. */
-      if (ui_but_is_interactive(but, true)) {
-        if (BLI_rctf_isect_segment(&but->rect, xy_a_block, xy_b_block)) {
 
-          /* execute the button */
+      if (!ui_but_is_interactive(but, true)) {
-          if (ui_drag_toggle_but_is_supported(but)) {
+        continue;
+      }
+      if (!BLI_rctf_isect_segment(&but->rect, xy_a_block, xy_b_block)) {
+        continue;
+      }
+      if (!ui_drag_toggle_but_is_supported(but)) {
+        continue;
+      }
       /* is it pressed? */
       const int pushed_state_but = ui_drag_toggle_but_pushed_state(but);
-            if (pushed_state_but != pushed_state) {
+      if (pushed_state_but == pushed_state) {
+        continue;
+      }
+
+      /* execute the button */
       UI_but_execute(C, region, but);
       if (do_check) {
         ui_but_update_edited(but);
@@ -1649,11 +1662,7 @@ static bool ui_drag_toggle_set_xy_xy(
       changed = true;
     }
   }
-          /* done */
+
-        }
-      }
-    }
-  }
   if (changed) {
     /* apply now, not on release (or if handlers are canceled for whatever reason) */
     ui_apply_but_funcs_after(C);