fix fallback behavior for unmodified GxEPD2

Issues exposed by https://github.com/meshtastic/firmware/pull/3356#issuecomment-1986950317

src/graphics/EInkDynamicDisplay.cpp

@@ -96,18 +96,45 @@ bool EInkDynamicDisplay::update()
 {
     // Detemine the refresh mode to use, and start the update
     bool refreshApproved = determineMode();
-    if (refreshApproved)
+    if (refreshApproved) {
         EInkDisplay::forceDisplay(0); // Bypass base class' own rate-limiting system
+        storeAndReset();              // Store the result of this loop for next time. Note: call *before* endOrDetach()
+        endOrDetach();                // endUpdate() right now, or set the async refresh flag (if FULL and HAS_EINK_ASYNC)
+    } else
+        storeAndReset(); // No update, no post-update code, just store the results
 
-#if defined(HAS_EINK_ASYNCFULL)
-    if (refreshApproved)
-        endOrDetach(); // Either endUpdate() right now (fast refresh), or set the async flag (full refresh)
-#endif
-
-    storeAndReset();        // Store the result of this loop for next time
     return refreshApproved; // (Unutilized) Base class promises to return true if update ran
 }
 
+// Figure out who runs the post-update code
+void EInkDynamicDisplay::endOrDetach()
+{
+    // If the GxEPD2 version reports that it has the async modifications
+#ifdef HAS_EINK_ASYNCFULL
+    if (previousRefresh == FULL) {
+        asyncRefreshRunning = true; // Set the flag - picked up at start of determineMode(), next loop.
+
+        if (previousFrameFlags & BLOCKING)
+            awaitRefresh();
+        else
+            LOG_DEBUG("Async full-refresh begins\n");
+    }
+
+    // Fast Refresh
+    else if (previousRefresh == FAST)
+        EInkDisplay::endUpdate(); // Still block while updating, but EInkDisplay needs us to call endUpdate() ourselves.
+
+        // Fallback - If using an unmodified version of GxEPD2 for some reason
+#else
+    if (previousRefresh == FULL || previousRefresh == FAST) { // If refresh wasn't skipped (on unspecified..)
+        LOG_WARN(
+            "GxEPD2 version has not been modified to support async refresh; using fallback behavior. Please update lib_deps in "
+            "variant's platformio.ini file\n");
+        EInkDisplay::endUpdate();
+    }
+#endif
+}
+
 // Assess situation, pick a refresh type
 bool EInkDynamicDisplay::determineMode()
 {
@@ -456,23 +483,6 @@ void EInkDynamicDisplay::checkAsyncFullRefresh()
     // It is only equipped to intercept calls to nextPage()
 }
 
-// Figure out who runs the post-update code
-void EInkDynamicDisplay::endOrDetach()
-{
-    if (refresh == FULL) {
-        asyncRefreshRunning = true; // Set the flag - picked up at start of determineMode(), next loop.
-
-        if (frameFlags & BLOCKING)
-            awaitRefresh();
-        else
-            LOG_DEBUG("Async full-refresh begins\n");
-    }
-
-    // Fast Refresh
-    else
-        EInkDisplay::endUpdate(); // Still block while updating, but EInkDisplay needs us to call endUpdate() ourselves.
-}
-
 // Hold control while an async refresh runs
 void EInkDynamicDisplay::awaitRefresh()
 {

src/graphics/EInkDynamicDisplay.h

@@ -68,6 +68,7 @@ class EInkDynamicDisplay : public EInkDisplay
     void applyRefreshMode();      // Run any relevant GxEPD2 code, so next update will use correct refresh type
     void adjustRefreshCounters(); // Update fastRefreshCount
     bool update();                // Trigger the display update - determine mode, then call base class
+    void endOrDetach();           // Run the post-update code, or delegate it off to checkAsyncFullRefresh()
 
     // Checks as part of determineMode()
     void checkInitialized();              // Is this the very first frame?
@@ -112,7 +113,6 @@ class EInkDynamicDisplay : public EInkDisplay
     // Conditional - async full refresh - only with modified meshtastic/GxEPD2
 #if defined(HAS_EINK_ASYNCFULL)
     void checkAsyncFullRefresh(); // Check the status of "async full-refresh"; run the post-update code if the hardware is ready
-    void endOrDetach();           // Run the post-update code, or delegate it off to checkAsyncFullRefresh()
     void awaitRefresh();          // Hold control while an async refresh runs
     void endUpdate() override {}  // Disable base-class behavior of running post-update immediately after forceDisplay()
     bool asyncRefreshRunning = false; // Flag, checked by checkAsyncFullRefresh()