I have spend some time to try to figure out under which conditions this is exactly happening. The sequence of events in the opening post does not seem to have been de cause.
I suspected a board package that I once installed to help somebody on the forum. This resulted in a clone Nano's port (old bootloader, CH340) being detected as a Thinary Nano (a 4808 based board). Package removed, issue still there.
I have tried to change board/port using the dropdown when creating new sketches with out conclusive results
I had about 5 Arduinos connected (Mega, Uno, Micro, Leonardo and SparkFun Pro Micro), disconnected them except for one. Issue still occured.
I've tried to observe the sequence of events in the status bar (right bottom) but it's difficult to keep an eye on the loading of the file, teh syntax highlighting and the recognition of a board at the same time.
I have also tried to limit the naming of my directories (not nesting too deep, not using the same basenames) to no avail.
The log files clearly show that two tabs with the same file are open but with different 'settings' for the innerWidgetState.
E.g.
"mainPanel": {
"main": {
"type": "tab-area",
"widgets": [
{
"constructionOptions": {
"factoryId": "code-editor-opener",
"options": {
"counter": 0,
"kind": "navigatable",
"uri": "file:///c%3A/Users/Wim/Documents/Arduino/gerard20221229/P1sender/P1sender.0.2.progmem/P1sender.0.2.progmem.ino"
}
},
"innerWidgetState": "{\"cursorState\":[{\"inSelectionMode\":false,\"selectionStart\":{\"lineNumber\":1,\"column\":23},\"position\":{\"lineNumber\":1,\"column\":23}}],\"viewState\":{\"scrollLeft\":139,\"firstPosition\":{\"lineNumber\":1,\"column\":1},\"firstPositionDeltaTop\":0},\"contributionsState\":{\"editor.contrib.folding\":{\"lineCount\":73,\"provider\":\"indent\",\"foldedImports\":false},\"editor.contrib.wordHighlighter\":false}}"
},
{
"constructionOptions": {
"factoryId": "code-editor-opener",
"options": {
"counter": 1,
"kind": "navigatable",
"uri": "file:///c%3A/Users/Wim/Documents/Arduino/gerard20221229/P1sender/P1sender.0.2.progmem/P1sender.0.2.progmem.ino"
}
},
"innerWidgetState": "{\"cursorState\":[{\"inSelectionMode\":false,\"selectionStart\":{\"lineNumber\":73,\"column\":1},\"position\":{\"lineNumber\":73,\"column\":1}}],\"viewState\":{\"scrollLeft\":0,\"firstPosition\":{\"lineNumber\":56,\"column\":1},\"firstPositionDeltaTop\":-16},\"contributionsState\":{\"editor.contrib.folding\":{\"lineCount\":73,\"provider\":\"indent\",\"foldedImports\":false},\"editor.contrib.wordHighlighter\":false}}"
},
{
"constructionOptions": {
"factoryId": "code-editor-opener",
"options": {
"counter": 1,
"kind": "navigatable",
"uri": "file:///c%3A/Users/Wim/Documents/Arduino/gerard20221229/P1sender/P1sender.0.2.progmem/telegrams.h"
}
},
"innerWidgetState": "{\"cursorState\":[{\"inSelectionMode\":false,\"selectionStart\":{\"lineNumber\":72,\"column\":5},\"position\":{\"lineNumber\":72,\"column\":5}}],\"viewState\":{\"scrollLeft\":0,\"firstPosition\":{\"lineNumber\":42,\"column\":1},\"firstPositionDeltaTop\":-17},\"contributionsState\":{\"editor.contrib.folding\":{\"lineCount\":74,\"provider\":\"indent\",\"foldedImports\":false},\"editor.contrib.wordHighlighter\":false}}"
}
],
"currentIndex": 2
}
},
@ptillisch, is there any way to get more debugging info out (e.g. grpc)? Not saying that I can find anything is that unknown territory 
But with all this randomness it's difficult to submit an issue on github.