Add window resize validation and document screen size limitations

- Verify that window actually resizes to requested dimensions
- Log warning when browser cannot resize due to screen size limitations
- Document that headless mode is recommended for reliable full-page screenshots
- Addresses reviewer concern about screen size limitations in non-headless mode

Author: certainly-param

src/SeleniumLibrary/keywords/screenshot.py

@@ -208,6 +208,14 @@ def capture_fullpage_screenshot(self, filename: str = DEFAULT_FILENAME_FULLPAGE)
         scrolling. It works by temporarily resizing the browser window to show
         the full page height, taking the screenshot, then restoring the original size.
 
+        *Important Note on Screen Size Limitations:*
+        When running in non-headless mode (regular browser window), the browser
+        may be unable to resize beyond the physical screen dimensions. In such cases,
+        the screenshot will capture only what fits within the maximum window size
+        the browser can achieve, and a warning will be logged. For reliable full-page
+        screenshots of tall pages, it is recommended to run the browser in headless
+        mode, which does not have screen size limitations.
+
         ``filename`` argument specifies where to save the screenshot file.
         The directory can be set with `Set Screenshot Directory` keyword or
         when importing the library. If not configured, screenshots go to the
@@ -256,6 +264,17 @@ def _capture_fullpage_screenshot_to_file(self, filename):
             import time
             time.sleep(0.5)
 
+            # Verify the window actually resized to requested dimensions
+            # In non-headless mode, browsers may be limited by screen size
+            actual_size = self.driver.get_window_size()
+            if actual_size['height'] < full_height * 0.95:  # Allow 5% tolerance for browser chrome
+                self.warn(
+                    f"Browser window could not be resized to full page height. "
+                    f"Requested: {full_height}px, Actual: {actual_size['height']}px. "
+                    f"Screenshot may not capture the complete page. "
+                    f"Consider running in headless mode for better full-page screenshot support."
+                )
+            
             # Now take the screenshot
             path = self._get_screenshot_path(filename)
             self._create_directory(path)
@@ -284,6 +303,17 @@ def _capture_fullpage_screen_to_log(self, return_val):
             import time
             time.sleep(0.5)
 
+            # Verify the window actually resized to requested dimensions
+            # In non-headless mode, browsers may be limited by screen size
+            actual_size = self.driver.get_window_size()
+            if actual_size['height'] < full_height * 0.95:  # Allow 5% tolerance for browser chrome
+                self.warn(
+                    f"Browser window could not be resized to full page height. "
+                    f"Requested: {full_height}px, Actual: {actual_size['height']}px. "
+                    f"Screenshot may not capture the complete page. "
+                    f"Consider running in headless mode for better full-page screenshot support."
+                )
+            
             # Take the screenshot as base64
             screenshot_as_base64 = self.driver.get_screenshot_as_base64()
             base64_str = self._embed_to_log_as_base64(screenshot_as_base64, 800)