source/blender/blenfont/BLF_api.hh

@@ -110,6 +110,12 @@ void BLF_size(int fontid, float size);
  */
 void BLF_character_weight(int fontid, int weight);
 
+/* Return the font's default design weight (100-900). */
+int BLF_default_weight(int fontid) ATTR_WARN_UNUSED_RESULT;
+
+/* Return true if the font has a variable (multiple master) weight axis. */
+bool BLF_has_variable_weight(int fontid) ATTR_WARN_UNUSED_RESULT;
+
 /* Goal: small but useful color API. */
 
 void BLF_color4ubv(int fontid, const unsigned char rgba[4]);

source/blender/blenfont/intern/blf.cc

@@ -335,6 +335,28 @@ void BLF_character_weight(int fontid, int weight)
   }
 }
 
+int BLF_default_weight(int fontid)
+{
+  FontBLF *font = blf_get(fontid);
+  if (font) {
+    return font->metrics.weight;
+  }
+  return 400;
+}
+
+bool BLF_has_variable_weight(int fontid)
+{
+  FontBLF *font = blf_get(fontid);
+  if (font && font->variations) {
+    for (int i = 0; i < int(font->variations->num_axis); i++) {
+      if (font->variations->axis[i].tag == BLF_VARIATION_AXIS_WEIGHT) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
 void BLF_aspect(int fontid, float x, float y, float z)
 {
   FontBLF *font = blf_get(fontid);

source/blender/editors/include/UI_interface_c.hh

@@ -2033,6 +2033,10 @@ void UI_init_userdef();
 void UI_reinit_font();
 void UI_exit();
 
+/* When changing UI font, update text style weights with default font weight
+ * if non-variable. Therefore fixed weight bold font will look bold. */
+void UI_update_text_styles();
+
 /* Layout
  *
  * More automated layout of buttons. Has three levels:

source/blender/editors/interface/interface.cc

@@ -6611,6 +6611,20 @@ void UI_reinit_font()
   uiStyleInit();
 }
 
+void UI_update_text_styles()
+{
+  if (BLF_has_variable_weight(0)) {
+    return;
+  }
+
+  uiStyle *style = static_cast<uiStyle *>(U.uistyles.first);
+  const int weight = BLF_default_weight(0);
+  style->paneltitle.character_weight = weight;
+  style->grouplabel.character_weight = weight;
+  style->widgetlabel.character_weight = weight;
+  style->widget.character_weight = weight;
+}
+
 void UI_exit()
 {
   ui_resources_free();

source/blender/makesrna/intern/rna_userdef.cc

@@ -314,6 +314,7 @@ static void rna_userdef_font_update(Main * /*bmain*/, Scene * /*scene*/, Pointer
 {
   BLF_cache_clear();
   UI_reinit_font();
+  UI_update_text_styles();
 }
 
 static void rna_userdef_language_update(Main *bmain, Scene * /*scene*/, PointerRNA * /*ptr*/)