docs: explanation for how to use debug env variable

docs/dev_notes.md

@@ -13,7 +13,7 @@ Then use a web browser to view the site indiciated, by default: http://localhost
 
 # How to build and view docs locally
 
-We have a CI action to presesnt the docs on github.io. 
+We have a CI action to present the docs on github.io. 
 To validate locally, you need the deps listed in `requirements.txt` installed. 
 
 Run
@@ -51,14 +51,15 @@ The CI runs with `--strict-markers` so any new marker must be registered in
 
 - the basic CI action runs the fast tests only, skipping all tests marked
   `visual` and `slow`
-- the CI action on PR runs the `slow` tests, but stil excluding `visual`. 
-- TODO: a new action for the visual tests is to be developed.
+- the CI action on PR runs the `slow` tests, but still excluding `visual`. 
+- a second action for the visual tests runs on PR.
 
 Check all tests are marked ok, and that they are filtered correctly by the 
 groupings used in CI:
 ```bash
 pytest --collect-only -m "not slow and not visual" --strict-markers --ignore=tests/visual_selenium
 pytest --collect-only -m "not visual" --strict-markers --ignore=tests/visual_selenium
+pytest --collect-only -m "visual" --strict-markers tests/visual_selenium/ -s --demo
 ```
 
 
@@ -97,7 +98,8 @@ pytest --cov-report=lcov --cov=src
 
 We use seleniumbase to test the visual appearance of the app, including the
 presence of elements that appear through the workflow.  This testing takes quite
-a long time to execute and is not yet configured with CI. 
+a long time to execute. It is configured in a separate CI action
+(`python-visualtests.yml`).
 
 ```bash
 # install packages for app and for visual testing
@@ -106,14 +108,15 @@ pip install -r tests/visual_selenium/requirements_visual.txt
 ```
 
 **Running tests**
-The execution of these tests requires that the site/app is running already.
+The execution of these tests requires that the site/app is running already, which
+is handled by a fixture (that starts the app in another thread).
 
-In one tab:
+Alternatively, in one tab, run: 
 ```bash
 streamlit run src/main.py
 ```
 
-In another tab: 
+In another tab, run:
 ```bash
 # run just the visual tests
 pytest -m "visual" --strict-markers
@@ -132,3 +135,17 @@ pytest -m "not slow and not visual" --strict-markers --ignore=tests/visual_selen
 Initially we have an action setup that runs all tests in the `tests` directory, within the `test/tests` branch.
 
 TODO: Add some test report & coverage badges to the README.
+
+
+## Environment flags used in development 
+
+- `DEBUG_AUTOPOPULATE_METADATA=True` : Set this env variable to have the text
+  inputs autopopulated, to make stepping through the workflow faster during
+  development work.
+
+Typical usage:
+
+```bash
+DEBUG_AUTOPOPULATE_METADATA=True streamlit run src/main.py
+```
+