[ci][playwright] Solidify flaky Playwright tests

This is an attempt at solidifying the playwright test suite, that has had testcases
intermitently failing. I have been lucky to be able to observe some race conditions
rather consistently locally, and so be able to debug them and solidify against them.

The race conditions are very slight - the Playwright test suite takes a screenshot
at every failure, and consistently I see the correct state on those, meaning that
the state is what it should be, by the time the screenshot is taken.

In consequence, I wound that, for a given flaky scenario, addressing one race
consition would often mask a second one, present in the same scenario. It's
possible that under a different execution context, more race conditions would
occur. Here are the race coditions I have found:

- When the Theia explorer view is created (at frontend reload or when explicitly
closed and then re-opened in a test case), it can take some time for its file nodes
to appear in the UI. Some tests were assuming that once the view was visible, they
would be too.
- Similarly, it can take some time for the Explorer's view tab to be created, after
the view is. Some test cases were checking immediately for the view tab being
visible, and failing when it was not.
- Also related to the explorer view, in at least one testcase, method "selectTreeNode"
was used to select a file node and assumed that it would be selected immediately after
clicking on it.

closes #12063

Signed-off-by: Marc Dumais <marc.dumais@ericsson.com>

examples/playwright/src/tests/theia-explorer-view.test.ts

@@ -29,6 +29,7 @@ test.describe('Theia Explorer View', () => {
         const ws = new TheiaWorkspace(['src/tests/resources/sample-files1']);
         app = await TheiaApp.load(page, ws);
         explorer = await app.openView(TheiaExplorerView);
+        await explorer.waitForVisibleFileNodes();
     });
 
     test('should be visible and active after being opened', async () => {

examples/playwright/src/theia-app.ts

@@ -109,6 +109,7 @@ export class TheiaApp {
             throw Error('TheiaExplorerView could not be opened.');
         }
         if (expectFileNodes) {
+            await explorer.waitForVisibleFileNodes();
             const fileStatElements = await explorer.visibleFileStatNodes(DOT_FILES_FILTER);
             if (fileStatElements.length < 1) {
                 throw Error('TheiaExplorerView is empty.');

examples/playwright/src/theia-explorer-view.ts

@@ -148,6 +148,10 @@ export class TheiaExplorerView extends TheiaView {
             await treeNode.focus();
         } else {
             await treeNode.click({ modifiers: ['Control'] });
+            // make sure the click has been acted-upon before returning
+            while (!await this.isTreeNodeSelected(filePath)) {
+                console.debug('Waiting for clicked tree node to be selected: ' + filePath);
+            }
         }
     }
 
@@ -231,4 +235,18 @@ export class TheiaExplorerView extends TheiaView {
         await this.refresh();
     }
 
+    override async waitForVisible(): Promise<void> {
+        await super.waitForVisible();
+        await this.page.waitForSelector(this.tabSelector, { state: 'visible' });
+    }
+
+    /**
+     * Waits until some file nodes are visible, for non-dot files
+     */
+    async waitForVisibleFileNodes(): Promise<void> {
+        while ((await this.visibleFileStatNodes(DOT_FILES_FILTER)).length === 0) {
+            console.debug('Awaiting for tree nodes to appear');
+        }
+    }
+
 }