Fix scaling factor for high-dpi displays

src/sdltiles.cpp

@@ -3624,6 +3624,38 @@
     return get_option<int>( "TERMINAL_Y" ) * fontheight;
 }
 
+// Measures scaling factor for high-dpi displays
+static std::pair<float, float> get_display_scale( int display_index )
+{
+#if SDL_VERSION_ATLEAST(2,26,0)
+    int x = SDL_WINDOWPOS_CENTERED_DISPLAY( display_index );
+    int y = SDL_WINDOWPOS_CENTERED_DISPLAY( display_index );
+
+    SDL_Window *w = SDL_CreateWindow(
+                        "probe",
+                        x, y,
+                        16, 16,
+                        SDL_WINDOW_HIDDEN | SDL_WINDOW_ALLOW_HIGHDPI
+                    );
+    if( !w ) {
+        return std::make_pair( 1.0f, 1.0f );
+    }
+
+    int lw, lh;
+    SDL_GetWindowSize( w, &lw, &lh );
+    int pw, ph;
+    SDL_GetWindowSizeInPixels( w, &pw, &ph );
+    SDL_DestroyWindow( w );
+
+    float scale_w = lw ? static_cast<float>( pw ) / static_cast<float>( lw ) : 1.0f;
+    float scale_h = lh ? static_cast<float>( ph ) / static_cast<float>( lh ) : 1.0f;
+    return std::make_pair( scale_w, scale_h );
+#else
+    ( void )display_index; // avoid unused parameter lint
+    return std::make_pair( 1.0f, 1.0f );
+#endif
+}
+
 static void init_term_size_and_scaling_factor()
 {
     scaling_factor = 1;
@@ -3658,7 +3690,11 @@
                                                      SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI | SDL_WINDOW_MAXIMIZED
                                                    ) );
 
+#if SDL_VERSION_ATLEAST(2,26,0)
+                SDL_GetWindowSizeInPixels( test_window.get(), &max_width, &max_height );
+#else
                 SDL_GetWindowSize( test_window.get(), &max_width, &max_height );
+#endif
 
                 // If the video subsystem isn't reset the test window messes things up later
                 test_window.reset();
@@ -3667,8 +3703,9 @@
 
             } else {
                 // For fullscreen or window borderless maximum size is the display size
-                max_width = current_display.w;
-                max_height = current_display.h;
+                auto [ dpi_scale_w, dpi_scale_h ] = get_display_scale( current_display_id );
+                max_width = dpi_scale_w * current_display.w;
+                max_height = dpi_scale_h * current_display.h;
             }
         } else {
             dbg( D_WARNING ) << "Failed to get current Display Mode, assuming infinite display size.";