Chrome since 63, including current stable and canary.
Windows 7.

1.1. unpack the attached extension to a top-level folder like r:\ext
1.2. go to chrome://extensions and enable developer mode, install the extension
1.3. click its "background page"

2.1. open devtools settings
2.2. click "Workspace" on the left
2.3. add the entire drive of the unpacked extension, not just its folder
2.4. close the settings, switch to "Sources" panel
2.5. click background.js on the left
2.6. don't close devtools

3.1. run a notepad app or any text editor outside of Chrome 
3.2. open background.js from the extension folder there
3.3. replace 1 with 2 in console.log(1)
3.4. save the file

4. switch back to devtools and observe

Expected: the edited file should be shown in devtools - with "2"
Observed: the old file is shown
          refreshing via F5 doesn't help

Workaround: close the file in devtools and open it again by clicking in the left subpanel

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Observed since Chrome 63 which enabled Persistence2.0 by default in devtools.
The actual problem was introduced earlier when the feature was experimental.
Bisected to 29f641736f1abdbd30ba57cd036774d5801bb236
"DevTools: drastically simplify UISourceCode.checkContentUpdated"
Landed in 57.0.2953.0
 

Deleted: test-ext.zip 486 bytes

test-ext.zip 486 bytes Download

Deleted: screencast.mp4 1.4 MB

	screencast.mp4 1.4 MB View Download