This QGS Python API script enables features in a vector layer to be filtered dynamically according to the layer's current visible extent on the map canvas.

QGIS_Dynamic_Filtering_Script.py

"""
QGIS Dynamic Filtering Script

Usage:
0. Ensure your vector dataset contains a ranking field, then load it into QGIS.
1. Adapt 'LAYER_NAME' and 'SQL_FILTER_TEMPLATE' in the 'Constants' section below.
2. Open the Python console in QGIS, then open and execute this script.
3. To disable filtering, copy and paste the following into the Python console:
   
   disable_dynamic_filter()

This script enables features in a vector layer to be filtered dynamically 
according to the layer's current visible extent on the map canvas. Only the e.g. 
50 highest-ranked visible features are displayed (depending on a customizable 
filter query).

It is assumed that there is a vector dataset containing a ranking field (e.g. 
'qrank'). Here's an example (no guarantee how long this storage will last): 
https://drive.switch.ch/index.php/s/RsYy9oBxWfad1am

"""

# Constants
LAYER_NAME = "castles_etc_ranked_osm_2025"
SQL_FILTER_TEMPLATE = """
SELECT * FROM "{layer_name}" 
WHERE "X" BETWEEN {x_min} AND {x_max} 
AND "Y" BETWEEN {y_min} AND {y_max} 
ORDER BY qrank DESC LIMIT 50
"""

def update_filter_on_extent_change():
    """Update the layer filter whenever the map canvas extent changes."""
    # Get the current map canvas
    canvas = iface.mapCanvas()

    # Get the current extent
    extent = canvas.extent()

    try:
        # Get the layer by name
        layer = QgsProject.instance().mapLayersByName(LAYER_NAME)[0]

        # Transform coordinates if needed
        layer_crs = layer.crs()
        canvas_crs = canvas.mapSettings().destinationCrs()
        if layer_crs != canvas_crs:
            transform = QgsCoordinateTransform(canvas_crs, layer_crs, QgsProject.instance())
            extent = transform.transformBoundingBox(extent)

        # Create the SQL filter with the current extent coordinates
        sql_filter = SQL_FILTER_TEMPLATE.format(
            layer_name=LAYER_NAME,
            x_min=extent.xMinimum(),
            x_max=extent.xMaximum(),
            y_min=extent.yMinimum(),
            y_max=extent.yMaximum()
        )

        # Apply the filter to the layer
        layer.setSubsetString(sql_filter)

        # Refresh the layer
        layer.triggerRepaint()
    except IndexError:
        print(f"Error: Layer '{LAYER_NAME}' not found")
    except Exception as e:
        print(f"Error updating filter: {e}")

def disable_dynamic_filter():
    """Disable the dynamic filter and restore full layer visibility.
    
    To disable the filter, copy and paste this function call:
    disable_dynamic_filter()
    """
    try:
        # Get the layer by name
        layer = QgsProject.instance().mapLayersByName(LAYER_NAME)[0]

        # Remove filter query
        layer.setSubsetString("")

        # Disconnect the extent changed signal
        iface.mapCanvas().extentsChanged.disconnect(update_filter_function)

        print(f"Dynamic filter disabled for layer '{LAYER_NAME}'")
        
        # Refresh the layer
        layer.triggerRepaint()
    except IndexError:
        print(f"Error: Layer '{LAYER_NAME}' not found")
    except NameError:
        print("Error: update_filter_function not defined")
    except Exception as e:
        print(f"Error removing dynamic filter: {e}")

# Store a reference to our function
global update_filter_function
update_filter_function = update_filter_on_extent_change

# Connect the function to the extentChanged signal
iface.mapCanvas().extentsChanged.connect(update_filter_function)

# Run it once to set the initial filter
update_filter_function()

print("Dynamic filtering enabled. To disable, run 'disable_dynamic_filter()' in the Python console.")