Actually do what the comment says.

Don't hijack forced partial updates (i.e., from UI elements) on always
FULL REAGL devices. It doesn't implode if we don't, and it makes for a
snappier UI.

frontend/apps/reader/modules/readerhighlight.lua

@@ -240,6 +240,12 @@ function ReaderHighlight:onHold(arg, ges)
             table.insert(boxes, self.selected_word.sbox)
             self.view.highlight.temp[self.hold_pos.page] = boxes
         end
+        --[[
+        UIManager.update_regions_func = function()
+            DEBUG("update ReaderHighlight onHold region", self.selected_word.sbox)
+            return {self.selected_word.sbox}
+        end
+        --]]
         UIManager:setDirty(self.dialog, "partial")
     end
     return true

frontend/ui/uimanager.lua

@@ -463,7 +463,7 @@ function UIManager:run()
                 waveform_mode = self.fast_waveform_mode
             end
             -- If the device is REAGL-aware, and we're doing a reader refresh (i.e., PARTIAL, but not regional), apply some trickery to match the stock reader's behavior if needed (On *some* devices, REAGL updates are always FULL, but there's no black flash)
-            if self.regal_always_full and not self.update_regions_func and refresh_type == UPDATE_MODE_PARTIAL and (waveform_mode == WAVEFORM_MODE_REAGL or waveform_mode == NTX_WFM_MODE_GLD16) then
+            if self.regal_always_full and not force_partial_refresh and not self.update_regions_func and refresh_type == UPDATE_MODE_PARTIAL and (waveform_mode == WAVEFORM_MODE_REAGL or waveform_mode == NTX_WFM_MODE_GLD16) then
                 refresh_type = UPDATE_MODE_FULL
             end
             -- If we truly asked for a PARTIAL, regional update, it's likely for an UI element, so fall back to the default waveform mode, which is tailored per-device to hopefully be more appropriate than the one we use in the reader