DeskTop: Fix volume icon flicker on launch with empty Disk II drives

Regression introduced in 05ef150 (new in v1.5); a bad branch when an
empty Disk II was detected caused the most recent volume icon to be
repainted.

Reorganize the function to use better flow control constructs.

Author: inexorabletash

RELEASE-NOTES.md

@@ -30,6 +30,7 @@ Project Page: https://github.com/a2stuff/a2d
 * Eliminate cursor flickering when clicking on the desktop or windows.
 * Time window open/close animations using VBL.
 * Add Apple+Escape as shortcut to clear selection.
+* Prevent volume icon flicker on startup with empty Disk II drives.
 
 ### Selector
 

notes/testplan.md

@@ -160,6 +160,7 @@ When steps say to a path e.g. `/TESTS/FOLDER/SUBFOLDER`, open the volume then ea
 * Open `TESTS/FILE.TYPES`. Verify that an icon appears for the `TEST08` file.
 * Open `TESTS/FILE.TYPES`. Verify that an icon appears for the `TEST01` file.
 
+* Configure a system with a SmartPort hard drive, RAMFactor card, and Disk II controller card with drives but no floppies. (MAME works) Launch DeskTop. Verify that the RAMCard volume icon doesn't flicker when the desktop is initially painted. (This will be easier to observe in emulators with acceleration disabled.)
 
 ## Selection
 

src/desktop/init.s

@@ -643,28 +643,27 @@ end:
         pha                     ; A = unmasked unit number
 
         jsr     main::CreateVolumeIcon ; A = unmasked unit number, Y = device index
-      IF A = #ERR_DEVICE_NOT_CONNECTED
+
+      IF ZERO
+        ;; Success! Draw it.
+        ldx     cached_window_icon_count
+        copy8   cached_window_icon_list-1,x, icon_param
+        ITK_CALL IconTK::DrawIcon, icon_param
+      ELSE_IF A = #ERR_DEVICE_NOT_CONNECTED
         ;; If device is not connected, remove it from DEVLST
         ;; unless it's a Disk II.
         pla                     ; A = unmasked unit number
         pha                     ; A = unmasked unit number
         ;; NOTE: Not masked with `UNIT_NUM_MASK`, `IsDiskII` handles it.
         jsr     main::IsDiskII
-        beq     done_create     ; skip
+       IF ZC
         CALL    _RemoveDevice, X=device_index
-        jmp     next
-      END_IF
-
-      IF A = #ERR_DUPLICATE_VOLUME
+       END_IF
+      ELSE_IF A = #ERR_DUPLICATE_VOLUME
+        ;; If duplicate, show a special error.
         copy8   #kErrDuplicateVolName, main::pending_alert
       END_IF
 
-done_create:
-        ldx     cached_window_icon_count
-        copy8   cached_window_icon_list-1,x, icon_param
-        ITK_CALL IconTK::DrawIcon, icon_param
-
-next:
         pla
         dec     device_index
     WHILE POS

src/desktop/main.s

@@ -67,7 +67,6 @@ JT_GET_TICKS:           jmp     GetTickCount            ; *
 ;;; Main event loop for the application
 
 .proc MainLoop
-
         ;; Poll drives every Nth time `SystemTask` does its thing.
         ;; At 1MHz on a //e this is about once every 3 seconds.
         kDrivePollFrequency = 35