avoid cursor jitter when updating progress display

When the progress display gets longer, and then shorter again, it causes
the cursor to jitter back and forth. Somehow I never noticed this until
this morning, but then it became intolerable to watch.

To fix it, pad the progress display to the maximum length it's occupied.

Sponsored-by: Svenne Krap on Patreon

CHANGELOG

@@ -16,6 +16,7 @@ git-annex (8.20210904) UNRELEASED; urgency=medium
   * reinject: Fix crash when reinjecting a file from outside the repository.
     (Reversion in version 8.20210621)
   * borg: Some improvements to memory use when importing a lot of archives.
+  * Avoid cursor jitter when updating progress display.
 
  -- Joey Hess <id@joeyh.name>  Fri, 03 Sep 2021 12:02:55 -0400
 

Utility/Metered.hs

@@ -459,12 +459,14 @@ updateMeter (Meter totalsizev sv bv displaymeter) new = do
 -- | Display meter to a Handle.
 displayMeterHandle :: Handle -> RenderMeter -> DisplayMeter
 displayMeterHandle h rendermeter v msize old new = do
+	olds <- takeMVar v
 	let s = rendermeter msize old new
-	olds <- swapMVar v s
+	let padding = replicate (length olds - length s) ' '
+	let s' = s <> padding
+	putMVar v s'
 	-- Avoid writing when the rendered meter has not changed.
-	when (olds /= s) $ do
-		let padding = replicate (length olds - length s) ' '
-		hPutStr h ('\r':s ++ padding)
+	when (olds /= s') $ do
+		hPutStr h ('\r':s')
 		hFlush h
 
 -- | Clear meter displayed by displayMeterHandle. May be called before